Nintendo has really put themselves in so many companies' good graces this generation it's crazy. I think it's also partly that these developers have seen over the past few generations that you can never count Nintendo out and they're always worth a decent gamble. It helps that for so many of these developers that it has paid off in spades. Of course, most of this is because they are releasing great ports that show they took the time to make it right and that they're showing respect Nintendo gamers and their expectations. We don't need games to look super amazing, but we do want the games to be fully featured and not buggy as hell.
I think it's more that the NS' hybrid form factor puts it in two markets and it has one market mainly to itself and the third party output retains value in this particular market even if a port is late and has compromises giving third parties more success than they expect as people are willing to get the NS version or even double dip due to portability.