I'd say the reason that we're up to our necks in 3D games is simply because of the law of supply and demand. Those tend to sell more units than do other genres, so that demand encourages other developers to follow suit. Presumably the fact that the 3D games sell better has much to do with the fact that platform games and the like have been with us quite a bit longer than have the 3D games, so they still seem more like the "hot new thing" to lots of folks. I think these things go in cycles though, and once everybody tires of an endless stream of 3D games, somebody will come up with a non-3D game that will sell like hotcakes. At that point, we can look forward to a glut of copycats for a while, and few 3D games.
That's my 2 cents worth ...