I think the main problem is that 1st party Nintendo games sell much more than 1st party Sony or Microsoft games, so 3rd party developers see much fiercer competition in Nintendo consoles to sell their games. When Nintendo decides not to develop that many games (Wii music, Animal Crossing Christmas) the third party failed to capitalize, so Nintendo pushed the first party games again.
Third party developers think they can't compete with Nintendo, so it's easier for them to abandon the console, and try to make it flop, than to indirectly contribute to Nintendo success and fight against Nintendo 1st party the next generations. It will happen to Sony if they keep developing strong first party games, cannibalizing 3rd party sales.







