It's supply and demand. Nintendo puts out new iterations of its biggest franchises because there is huge demand for them. If you don't like the games that's fine but apparently the market can take the number of "rehashed" games Nintendo releases and even wants more of it.
Also, it's not like Nintendo wasn't developing new IP's. They are, these games just don't sell as well as Mario and Pokemon.
And to add one more thing: The percentage of "rehashed" games Microsoft and especially Sony release is ever increasing. Why is this? Because they are safe to develop and the market wants more of them. The only reason God of War (for instance) feels fresher than Mario is because... it is. I really want to see those same people complain about the next God of War, the next Forza, the next Call of Duty (yeah, some people complain about that one - because it sells so well and they can't take that), Assassins Creed, Mass Effect, Bioshock or Ratchet and Clank. One day these franchises will have just as long of a history as Mario - remember that when you complain about Nintendo's games.
If I like a franchise I want more of it. I want more Mario and Pokemon, more Bioshock, more God of War, Uncharted and Call of Duty. I don't want a new game every year but one game per generation (2D Mario) or bi-annual releases are fine as long as the quality stays high. That's because I'm a gamer. I can appreciate new and old franchises and when millions of people like a series I won't complain and tell the developers to stop making it. That's just insane and incredibly selfish.







