The question isn't "How does Nintendo convince their fans to buy 3rd party games," nor is it "How do 3rd parties convince Nintendo fans to buy their games." What you need to be asking is "How does Nintendo convince 3rd parties their system is a viable and worthwhile investment to properly develop games for?" And yes, that is entirely on Nintendo. It's not the third party's job to establish an audience. They don't need Nintendo, per say. All they need is an audience and a platform that either has the established userbase already, or one which is strongly cultivating it.
The first post got it right. In order to do that, they need to diversify and start making games for the same audiences as whom 3rd parties target. They need to market the system at that audience to show their system is viable in terms of the games they want to play (shooters, racers, sports games, RPGs, etc), and they need to build a strong online community. Doing so would prove to 3rd parties there is an established audience on the system hungry for the games they make. The 3rd parties can tap into that easily and effectively with their established games.
