2D is expensive, hard to do, time consuming, and to top that off isn't all that popular anymore for the general gaming populace.
You can't easily change anything like you can with 3d models. Say you wanted to change the hairstyle of a character, you would have to redraw/modify by hand every single frame. While with the 3d model, you just adjust the polygons on the model and you've just changed them for every frame. Or if you want to provide an alternative costume for a character, well, that's redrawing a bizillion frames again, at least with 3d you can re-use the skeletal animation data. Which is why in Blazblue you can only adjust the colors of your character and not anything else about them.
Take Blazblue even, the backgrounds are all done in 3d. To get that kind of parallax depth effect in pure 2d would be ridiculously hard if not impossible, in 3d, its done automatically for you by the mathematics involved. Or watch when the camera flies out from the background and you see the character's up close and pixelated, that never happens in a 3d game.
2D actually starts taking up a lot of space, as 3d data is far more compact, as you only have to store the positions/orientations of your bones per frame, while in 2d, you need an entire sprite per frame. When you start scaling up to high definition resolutions, these sprites start taking a lot of space and memory.
The animation in Blazblue is actually quite crude if you go into a replay and step through it slowly. I know when I switch between Blazblue and SC4, SC4 feels a ton smoother to me. Sure you could animate a full 60fps animation in 2d, but, you do want to actually play the game sometime?
Gameplay wise, there's no reason that 2.5D can't play out identically to the equivalent 2D game. At its heart, Blazblue is really just a bunch of bounding boxes moving around the screen, the representation of that could be anything. If you added a playable 3rd axis and turned it into a true 3D fighter, then yeah, you'd be talking a totally different beast and likely lose the soul of the gameplay in the game (which is likely why the 3D MK games failed, I've never played any of them, but, that really gives it a totally different set of game mechanics that their original audience probably didn't want).
If you went the other way, and say Namco made SC5 into a 2D fighter, could you really tell me that would be an improvement for the better and wouldn't destroy/fracture their fanbase? Course, if you played Raphael, would you even notice? :)
