The GCN actually had a different architechture to the PS2/Xbox. It led to some games being axed before release (Max Payne anyone?) and a fair few of the games that did make it across ran like shit (Hitman 2?) despite coming out many months later.
There are multiple reasons for games selling worse on one format than another. But I do think 3p's screwed over the GCN using games launching 6 months later, then not selling well as the reason not to make sequels available on GCN. But why would you expect a late port to do anywhere near as well?
But yeah, Wii U is weaker than PS4 or XBONE, doesn't stop them doing a PS3/360 port though (Tomb Raider etc.) on a purely technical front it's possible. However, the awful sales that games such as FIFA and Need for Speed saw, means most 3rd parties know a Wii U version of a game is dead money right now.
It's just not intrinsically linked to the GameCube sales of 3rd party games.
RIP Dad 25/11/51 - 13/12/13. You will be missed but never forgotten.
