The game required more, and the gameplay was actually quite good, but it wasn't able to retain an audience. Like you said, you stopped playing it waiting for more - more of what? the gameplay should be able to just hold you in itself. Several games, particularly on PC of all places, launch barebones & grow out from there. LoL launched with a single gameplay mode, tiny cast of characters (relative to its current roster & its competition when it first opened its beta), and featured a portal that didn't really work, but its gameplay was able to retain an audience, and that game didn't even have a 'training room' - you just had to play the game competitively.
The core game-play was indeed good, but the game was very limited. As I noted before, the game had only 6 characters (only one of which appealed to me, personally), which meant that you were very limited in what characters you could play as and against. Furthermore, as I noted the game only had one stage, which you became intimately familiar with in both VS and in training mode. Additionally, the the game never had an interesting aesthetic to begin with and the planned cosmetics system (which was to fund the game, long term) never made it into the alpha. And while the developers had begun to add alternative moves to the game, allowing you to tailor your move-list as you saw fit, the game lacked the infrastructure to allow you to do so
after you knew who your opponent were playing, forcing you to choose conservatively rather than by utility. And, importantly, since the game only had online, ranked VS, you could not play "casual" matches with people you actually knew, online
or offline. So I could not play with the small online community I was hanging out with at the time, unlike every other game we were playing. So I decided to wait, to wait for more characters, more stages, cosmetics, better match infrastructure, and support for lobbies. Much like SFV, the problem was not the game-play, but everything around it.
You also mention LoL, but I doubt that LoL was quite as limited as RT when it first appeared. For one thing, LoL was actually released, unlike Rising Thunder which never left alpha! You also mention the that LoL initially had a "
tiny cast of characters (relative to its current roster", which seems a bit disingenuous, considering that this "tiny" cast spanned some 40 characters as far as I can tell. That's not "tiny", even if they added many more later, and it is certainly much more than the 6 characters Rising Thunder launched with. Not to mention that LoL was built off the back of a game that had already established a niche within RTS games, while Rising Thunder was trying to create a new niche within fighting games. They are not really comparable at that stage.
So yes, Rising Thunder did fail at both capturing a larger, more casual audience, or retaining the audience it did appeal to. Maybe that isn't rejection, but it certainly wasn't successful.
That is an odd conclusion, considering that the game was never even released, and was never widely marketed outside of the FGC. And it was not shut down because of any failure on the part of the game nor on part of the developers, but because Riot seemingly saw enough potential in that formula to acquire the company.