I think that for this generation at least a lot had to do with the fact that 3rd party developers could make one multi platform game which could run on 360/PS3/PC while they would have to make a unique Wii version due to far less capable hardware. It simply wasn't cost effective.
Consequently the bulk of development resources were dedicated for those platforms and the Wii was left with the scraps so to speak. All of this leads to a situation where most 3rd party games are perceived to be "inferior" on the Wii even when motion controls are used effectively.
To answer the OP more directly I would say that people saying only Nintendo games sell well are misinformed but only to an extent. While it is true that Nintendo 1st party games sell the best overall, there have been some successes by 3rd parties as well. Of course this pales in comparison to how well they perform on the other consoles and this is what most people focus on.
Hopefully Nintendo learned its lesson and will at least provide a comparable console to the 720/PS4 otherwise 3rd party support may turn out just as bad long term.
