Let's face it. Nintendo hasn't had the best third party support since the SNES. Why? What happened?
Wii: Graphically underpowered. Third party devs would rather sell to the combined PC + 360 + PS3 owners than take a chance making their game for a single system. Wii sold well, but active user base was very low. Very little third party support.
N64: Carts were expensive and couldn't hold as much memory. Cutting edge graphics. Low sales compared to PS1. Nintendo pissed devs off with tyranical bussiness practices from the NES/SNES days. Very little third party support.
Gamecube: Tiny Disks were a pain to make games for. Sales of the console were low. Best third party support after NES, and SNES.
NES/SNES: Graphically competitive. Good sales. Easy to develop for. Fantastic third party support.
So what does Nintendo have to do to get good third party support? Simple. Make a system that is graphically competitive, sells well, and is easy and cheap to make games for.