I don't think it's just a matter of "the fanbase" not being there. If anything, the fanbase probably doesn't care about games like this because it's not Smash. Smash is the king of the category it created AND has all the nostalgia appeal.

The other big factor, I think, is that games like Brawlout make the most sense as local 4-player experiences with gamepads. PCs are not well-suited for that. PCs are about keyboard & mouse and online play mostly. Maybe local 2P. But getting 4 gamepads working properly on a PC is a pain in the ass and just something people usually don't even think of going through, even if computers have never been as gamepad-friendly as they are today.