I've played SFIV, it's a decent game, but I still think 2D is better for fighting games.
SFIII and KOF had flashy graphics, while still being 2D.
With 3D games, you can't really see eveything that's happening, at one time- the camera has to switch viewing angles all the time, to try to let you see everything that's going on in the fight. Which isn't the case with 2D fighters- you can see everything that's going on between the two fighters, all of the time, with no delay. Which makes it easier to play your best game.
Two other reasons Capcom stuck with 2D, was that they really believed it was better for fighters (hence SFIII was 2D), and just like SNK with their Neo Geo, Capcom's programmers were masters of 2D programming, but not 3D. From what I understand, programming in 3D is very different to 2D.