There is only one reason that 3rd parties will ignore the Wii - because mjc2021 said so, and his word is infallible. ;P
Well in this particular case... I'd like to think there some validity to what I'm saying.
It does and it doesn't all in the same. What you need to look at is the same game on all 3 platforms. Who's to say how well Assasin's Creed would have done on the Wii, or how Ravving Rabbids would have done on the PS3?
If you line up the games that came out for all 3, you'd have an argument with a better foundation.
For example, we know clear well that for COD:WaW, it was 360>PS3>Wii, for Guitar Hero 3 it was Xb360 4.23M >Wii 4.21M > PS3 1.89M .... so right there, we have a bit to both prove and disprove your argument. This would make a much better point to balance out and see what 3rd parties have profited from Wii development and which have faltered.