I think a lot of it has to do with devs not taking risks anymore. I mean back in NES generation had all these wonderful and new kinds of games we'd never seen. I mean does anyone realize how groundbreaking Zelda and Metroid were for their time. And this kinda continued upwards until the ends of the 5th generation. 6th generation and onward it might be because everything has been done or devs are getting comfortable but you are not seeing that crazy innovation anymore. I mean sure everynow and then we get a LittleBigPlanet or a Spore or a Wii Sports but not as much as we used to. Hell we are lucky to get innovative next steps up from existing series such as Metroid Prime.
Eventually though this will be forced upon devs considering over saturation of a market with the norm is how the market crashed in 83 in America.