It's just this forum. Neogaf isn't representative of mainstream gaming. Just take a look at Steam stats and see what's really popular, here I'll do it for you - top 5 are Dota 2, Counterstrike, PUBG, PayDay 2, H1Z1 and then its GTA V, Ark, Civ V. None of those besides GTA V get talked about much here.
Yakuza 0 sales in the west are 'niche'.
Some fans also think it wasn't helped by having ONLY an english localication in europe either, where a lot of games have at least four or five different languages.
Yakuza 3 came out with a lot of bad press when content was cut out making its 'fans' mad, if its fans were upset, I doubt others would buy it. Sales numbers for Y3 meant it also hurt Y4 when it came time for retailers to set order numbers. Dead Souls probably killed the franchise for a while in the west. Yakuza fans got lucky when Sega decided to try again with Y0, Y:Kiwami and Y6 and I'm guessing as a result they also decided to do Y5 but just digitally (again pissing off physical buyers!).
I've had my eyes on the franchise since 2 but only got into it with 4...years after it's western release.
The first game I admit, just completly went past me, never heard of it when it came out. The second game did catch my attention but then I wanted to play the first game first, and it kinda annoyed me that the first game was english dubbed while the second was subbed, so I didn't bother in the end.
Then 3 came, and I was determined to finally get into the series...until, like you said, they decided to cut a bunch of quests and other side content from it's western localization. Even to this day, now that I'm into the series, I don't want to touch the game in it's current state.
Then Sega made it's biggest mistake...they released HD ports of 1 and 2 for the PS3 in Japan and I thought "Great, I can finally get into the series now once they come out in the west!", because the translations were already done and I thought releasing them over here was a no-brainer.
...but they never did. Huge missed opportunity.
When 4 came out over here I had faint interested but ultimately felt it was too late for me.
It's only thanks to Binary Domain that I still gave the series a chance, loved the game, learned that it was by the Yakuza development team which made me decide to fuck it and just get Yakuza 4 already. It was instant love, even if I never finished the game due to pacing issues with the character switching.