Series in which you went from rabid fan to having a casual interest. What lost you?

Mortal Kombat I guess. I played a lot of MK when I was a kid, but that was mostly because of wanting to see all the fatalities and to enjoy the brutality (and to see who's better against friends ;)). I still like the series, but I bought the last two entries mostly out of a sentiment and didn't play too much of any of them.
Mass Effect.

Mass Effect 3 happened..

Final Fantasy

Final Fantasy XIII happened. Together with Final Fantasy XII-2 the only FFs I havent finished. Don´t own a PS4 so haven´t played XV yet, can´t say that I´m intrigued to though..
Pokemon. I was sorely dissapointed by Pokemon X/Y. After that I gave the omega ruby a try, couldn't finish past gym 3 and never end up playing Sun/Moon.

I'm still interested in the franchise and hoping that the franchise moving to Switch will give me enough reason to play the next entry.

Oh, and PES. Because friends that I usually played the game with are now so busy, we rarely play together anymore.
I went from being Jawmuncher levels of loving the Dead Rising series to hating it due to all the awful changes they made with 3 and especially 4. I've never seen another franchise nosedive so hard due to such impressive levels of incompetence and stupidity.
Sonic, though for me i started to cool on the series starting with Unleashed. Loved Secret Rings, was ambivalent on Shadow, loved Heroes, SA2 is #1, so my Sonic taste is iconoclastic.
I used to be a huge FF fan, starting with the first American release, on to FFIV, VI, VII, VIII, and so on.
I am pretty much in this same realm too. I want to go back and speedrun through FF4 again, it's that much fun.

But when I try to talk myself into looking at their newer FF IP, I just can't. There is some kind of charm their old FF IP has that I am just really attached to. That being said, I am excited to bust out FF6 on the SNES classic come October.

The transition to dota 2 was pretty hard. it required a higher spec pc to run as smooth and also lacks a lot of heroes.

it was also the time when League was becoming popular. It can also run on a potato.
Halo - Halo 4 happened. Then the MCC debacle. And inbetween the two was the Xbone reveal incident, which capped a push that resulted in myself and most friends skewing toward PC and going with PS4 for our must-have console game at that point (Destiny).

Mass Effect - I played through the original at least half a dozen times, and did three, maybe four, playthroughs of ME2. Once I finished 3, all motivation was literally sapped from me. I didn't start my Renegade and Insanity playthtoughs immediately after as usual... put it on the shelf for a few months, eventually tried to start an Insanity playthough, didn't make it far. Never touched any of the dl, though hear Citadel's great. Still haven't gotten to Andromeda.
I used to love the Halo series but I stopped following it after Reach. My friends stopped hosting local multiplayer sessions and after that I gradually lost interest. I won't mind playing every now and then but I was never too interested in the story after Reach to follow the series myself. There are too many other great games out there to take up what precious little free time I have than struggle through what's turned into a mediocre space opera.
Street Fighter V, because of its greater emphasis on revealing skin and its lack of single-player content.

Conversely, I love previous SF enough that I wrote the liner notes for the SF2 Movie Blu-ray and own several SF series entries. I even used to own Zero 3 for Saturn, imported it shortly after getting the hardware circa 1999.

Edit: MGSV had kinda the same issue for me. Recasting, Quiet... Enough to turn me off. Loved 4 in spite of its flawed segments though because it wrapped up the story nicely, even if it was more than a bit convoluted.
Souls series for me.
I remember when I was all over Dark souls 1. But spending too much time with the "souls community" made me want nothing to do with it anymore. Bloodborne was one of the most dissapointing gaming experiences. Couldn't even finish Dark souls 3 because it felt so hollow
pun intended
Somehow, I'm still hanging on with Square Enix even though the last decade has been nuts.

As for a series I became a casual fan in, I'd say Metal Gear. Only because I bought V and haven't opened it. No idea why, either. Just never did. I really liked 4 too.
Tales series. Used to be a fanboy of that series. After Vesperia, I was losing interest though. Somehow the games felt even weebier and more cliche than usual, but that's prolly around when I noticed how much it follows anime trends.
Final Fantasy. Day one for pretty much everything up to 13. However, I didn't enjoy 13 near as much and the wait for the game diminished my interest. Then with the focus of pumping out sequels for 13 for a bit and the lack of information on 15 for a long time, I just lost the interest I once had.

grew up loving the anime, the TCG, the first 2 generations. i was a pokemon fanatic.

once generation 3 hit, my interest in it just fell off. still i played the games but stopped watching the anime when generation 4 hit i started losing so much interest in the series etc. once generation 5 hit, it was all over. I can't remember a single more than 3pokemon from generation 5. my interest in it died then. But my little brother got into pokemon recently so I still buy the games for him and help him if he needs any help with it

maybe i'll get back into it with the switch game
MGS series. With the way David Hayter was treated poorly by Hideo, I lost interest in the storyline of MGS5 and the series as a whole. Hayter voicing snake was one of the best thing about the series for me.
Tales series. Used to be a fanboy of that series. After Vesperia, I was losing interest though. Somehow the games felt even weebier and more cliche than usual, but that's prolly around when I noticed how much it follows anime trends.
Vesperia was the last Tales game that I really enjoyed. The rest had some fun mechanics but the stories became way to anime cliche.
Mario Kart. It's just not fun being pelted with items anymore, especially when I'm so confident in my racing skill. Playing without items is boring, so the only way I get enjoyment is from winning 1st place through all the chaos.

If I play Mario Kart, I'm only playing with friends from now on, or random online battles thanks to MK8DX making battle mode fun again.

Also, despite the awesome non-Mario franchise representation in the game that I never knew I wanted more of, the Mario franchise character selection in MK8 was a real downer for me.

Tales series. Used to be a fanboy of that series. After Vesperia, I was losing interest though. Somehow the games felt even weebier and more cliche than usual, but that's prolly around when I noticed how much it follows anime trends.
I don't consider myself a Tales fan, but I played the hell out of Symphonia: four playthroughs total, last three playthroughs done in a row. It's one of my top favorite games of all time. I wanted to play more like it, so I played Vesperia next and loved it. Then, I played The Abyss next and enjoyed it. Those are the only 3 Tales games I played. None of the others interest me enough to play them, but I do like watching no-commentary playthroughs of them for the stories and skits.
Resident Evil series

Resident Evil 4. Now 4 is an amazing game, don't get me wrong. But two things happened:

1. It killed the Resident Evil story, which never recovered
2. It switched from the static survival horror I loved to TPS. This probably saved the series and overall it's regarded as one of the greatest games ever made, so they definitely made the right choice. It just wasn't what I wanted from Resident Evil.

Granted, I thought RE0 sucked, so I guess that's when I first started questioning the series.
Call of Duty, for sure. The early CoDs are still some of my favorite FPS campaigns, but I was pretty much over the series after the post-MW2 Infinity Ward exodus.
I was a real big Hitman 2, Contracts, Blood Money fan. Real big. Then Absolution happened and then I wasn't. This meant I didn't get into Hitman (2016) as much as I probably would have if Absolution hadn't dampened me to such a degree. If the 2016 version happened after Blood Money it would probably be my favourite game of all time.

Also Far Cry 2 was so good but then 3 and 4 and Primal all happened and I don't care. I still love the first Mass Effect but every game that came after dampened me a little. Every time I think hell yeah I'll replay Mass Effect I then remember the rest. Not bad games just not what I was hoping how they would turn out.
Has anyone mentio...oh they have but time for my stories.

Pokemon: Loved gen I, imported gen II. lttp on gen III but Colosseum bought me in and I got a GBA too. Gen IV I bought a DS to play and ugh...DP were just so slow in-battle and if you thought breeding is bad now it was a lot worse then and that was a big part of the postgame. That taste of Gen IV was so bad for me I skipped HG/SS and Gen V. After playing XY I went back to Black and saw it was the better single player story. I still completed the full Pokedex (non mythicals) in XY and have some meaty Battle Maison teams. For SM I completed it, completed Alola dex and have not gone back since. Unsure about Ultra SM.

Fire Emblem: I used to be someone that played the games as soon as they were out in Japan. What changed? Shadow Dragon and the fan community. Yeah the community has always been a tad argumentative but I felt tier lists were starting to become actual trolling and I decided to contest an in-joke being bottom every time (even for games they were not in) and getting lol as a response I just thought my time is not worth this bullshit (I should note with a wave of new fans following awakening the fan base continues to change). That said for the 3DS entries I've been there on European release date (conquest only for fates and it took me a few months to start playing it), tried and dropped Heroes pretty early on. So I probably still have more than a casual interest but no longer rabid about it at all.

The Legend of Zelda: I was introduced to the franchise in it's Renaissance period of OOT, LA:DX, MM, Oracles all in short succession and what followed were some good but inferior games in the Gamecube era which brewed a bit of apathy for what followed. There was then a long wait for Skyward Sword that I just could not get into. Did I forget the DS games? Guess I did. What about 3DS? Can't fault the ports; Link Between worlds I've not finished yet and the online demo of Tri-force heroes was a chore. Breath of the Wild is an interesting crossroad and a test (you can essentially try to beat it whenever you're ready) and I'm not sure where Nintendo goes from there.