Honestly, I'm getting tired of everyone whining about how the games are the same. Many games have tried changing their ways when they got old, and look how it usually turns out. Splinter Cell wasn't as good when they tried mixing it up after Chaos Theory. Crash Bandicoot tried changing its ways after Naughty Dog sold it, and those games are mere shadows of their former glory. Final Fantasy has stepped out of their element a few times, and quite often (Dirge of Cerberus, for example), it fails horribly.
In addition, Nintendo isn't as bad at that as we tend to say. Consider some of the other Nintendo classics: Mario has plenty of different kinds of games, the best examples of diversity being the RPGs (Paper Mario, Super Mario RPG, Mario and Luigi, etc.). Metroid had a fantastic transition from 2D to 3D. Even Zelda tried switching things up a bit on a couple occasions, both of which failed (the side-scrolling battles in Zelda II were no good at all, and I won't even go into the Phillips CD-i abominations...).
All that to say: stop complaining. They're sticking with what's good because, though it's kind of redundant, it's what we know is good. Familiarity with novelty: Nintendo does that rather well.