This is a trend that gamers have seen picking up speed since the first 3D graphics cards were made for PCs. Unlike those days, however, 3D graphics are no longer ugly and boxy, and the level of detail available for 3D models can sometimes even surpass what can be rendered by 2D sprites.
Technology advances, and the old stuff just doesn't do as well as the pretty new stuff in the marketplace. Just look at movies: How many hand-animated movies are released in theaters these days? Now how many CG kids' movies? Whatever happened to claymation? Do people still buy movies on VHS? Technology moves forward, and the newest trends quickly become the standard, pushing the old stuff out of favor.
Personally, I still love 2D games. Be they true modern 2D (BlazBlue comes to mind, as do the Monkey Island updates), retro 2D (a lot of indie games use 8-bit style graphics), or 3D on a 2D plane (Bionic Commando: Rearmed, Shadow Complex, and Sonic 4), I think that 2D games are just as valid now as they ever are. However, just like claymation, they're becoming a niche thing, and not the eye-catchers that they used to be.