I think the FPS can (in part),be put to the "supply & demand" angle Chuckles mentioned,but I also think that it can be put down to the basic PC design as well.
I often recall the questions years ago of why the Amiga lacked it's own "Doom"/Wing-Commander-esque sims game-clones,(Prior to Alien Breed 3D,not that I've seen it personally).
PC's always had that advantage of quick graphic screen writes that made manipulating 3D so much quicker,add on CPU power & it always beat systems like the Aimga hands down once they hit VGA modes,486's etc.
But, the PC's are & always have been seemingly hopeless at the old sprite,screen scrolling hardware that those old Amiga. 16bit consoles,heck even C64's always seemed great at in 2D.
But I agree it seems to be all driven by the "corporate, men-in-suits" mentality image that drives the "industry" today. Just look at how many talanted home/cottage programming teams from the Amiga,C64 16bit/8bit times are still about today?
There were some terrific,original designers,but not sadly "business-savvy",so they ended up being forced out.
I remember reading about the "death" of the classic (&2D)graphic adventure format,that one of Al-lowe's (of Leisure Suit Larry fame),programming friends put up, in relation to Al-lowe's sacking (along with several other longtime employee's at Sierra) to make way for these FPS type games.
It's all well & good to say it's due to sales but if you only ever have the choice of McDonalds, who's to say you won't like Taco's or pizza's or pasta,or chinese if they're not on the menu,(If you take my meaning.). Problem is,who's going to publish a 2D game that'll get criticized for being 2D?