Why do people claimed only Nintendo games sell on Nintendo platforms as if a bad thing?

[bananaking21]

Kenology said:

bananaking21 said:
because if only nintendo games sell on nintendo platforms then third party games wont sell and that equals no third party games, therefor most of the games that come out wont come out on the system (example the Wii)

Wii sold a shit ton of 3rd party software.

yet third party sales on PS3 and Xbox are much higher.... i just used Wii as in example to explain a point, nothing more