Nintendo makes new stuff frequently, it's just that when you have franchises that have lasted going on thirty years or more and thus they've had time to accumulate many entries and spinoffs, it makes it SEEM like Nintendo uses only those franchises.
Old franchises always tend to receive more media coverage and enjoy better sales, precisely because they're so familiar.
Most franchises are lucky to not burn out before the decade mark and yet Mario, Zelda, Metroid, Fire Emblem, Pokemon, Kirby, Donkey Kong, and more are not only still around, they're still relevant and win critical and commercial acclaim all the time. I fail to see the logic here.
Then again, Team Ninja proved with Other M that they excel at burning decades of accumulated goodwill at record speed.