Why not just stick with making 2D games look as good as possible?
Cost of development, versus actual sales?
Making a 2D game in this era, outside of portables (
and that may change come March), is suicide.
I'm perfectly willing to agree to play games that play like 2D sidescrollers and fighters, so long as developers are willing to agree to stop screwing up games with a good legacy like Castlevania by making it into some slipshod 3D action game. Not to say that a good true 3D Castlevania couldn't be made, but seeing as how developers just bite the existing conventions and tack their name on to it, it's unlikely that I'll ever see the sort of 3D realization of some of the games I'd like to see.
They could strike a happy medium by just using polygons instead of sprites.
However, I would like to see a true 3D Street Fighter at some point. Just to give the fighting game genre a swift kick in the pants. I'm tired of the VF derivatives that dominate 3D fighting.
I don't think it's so much a question of why not keep making 2D games with sprites, so much as if you can make a 3D game look like a 2D sprite based game, what's to stop you from doing it?
One of the things I would love to see in Street Fighter are different outfits for the fighters, and that's something that will never happen if Capcom has to draw a new sprite for each character.