It mostly stems from Nintendo relying on Mario, Zelda, Donkey Kong and other classics to sell their consoles every time. In terms of new IP's and in-house support and effort, Nintendo falls far behind MS and Sony for the past three generations, the number of games that feature Mario is right around 200, which is staggering.
An good example of changing it up a little bit would be developers like Naughty Dog; they made Crash Bandicoot for the PS1, Jak & Daxter for the PS2 and Uncharted for the PS3, this kind of effort and variation from developers on Nintendo platforms is unheard of.
The real question for me is; do I mind Nintendo's focus on Mario and Zelda and other old greats? No, not one bit.
Check 3DS eshop for some great new Nintendo IPs.
Pushmo/Crashmo, Dillon's Rolling Western, Sakura Samurai, Ketzal's Corridors, FreakyForms, etc..
I've never stated that new IP's don't spring up on Nintendo platforms, just that they do with much smaller frequency.