well my theory is,
right look at the nes, mostly its games consited of platformers sidescrolling beat em ups, rgs, and basically games in which a controler is neccessary.
Playing for example mario with a keyboard is a very diferent expereance (emulators for those who are wondering)
Now look at todays games, im sad to say they are mostly fps games from what i can gather (not saying theres anything bad with fps games, but its just my oppinion) with less of any other genre being produced. And since the console fps games evolved using the gamepad, im assuming they are still stuck in there roots, or have evolved to use the gamepad efficently enougth to justify the non use of keyboards for specific genres, which would more than likely make some other games somewhat akward to play in general compaired to controlers (yes some games are easier to play with controlers rather than a keyboard, my oppinion no need for a argument over it).
Oh and also its hard to justify buying a keyboard for a console from a companys point of view, since of course keyboards are available everywhere, and well they wont get a profit from them, where as gamepads are console specific, and with this in mind they can charge whatever the hell they like for them without having to many competitors to deal with and since consoles usually come with the manufacturers controler most people assume that its the best one to buy.
okay okay it wasnt just one theory, i got somewhat bored.
sorry if this doesnt make any sence.