@Kiryu-Chan
Actually, WiiU couldn’t play GameCube game software, and the Wii software included was apparently not that expensive to include, as it just emulated the console digitally. (And frankly the WiiU was still cheaper then the competition at the time.)
Also, the WiiU actually was said to be easy to develop for, (judging by all the Indie games on it’s Eshop) but many consumers & developers scoffed at it’s “Game Pad” “gimmick,” and some like EA & the creator of Minecraft bashed it for not being as powerful as a PS4, or another “gimmicky” Nintendo system, and such…
The name probably hurt as well, as haters of Wii, refused to give it a chance wrongly assuming it was just another “casual” Wii system, and meanwhile the more casual Wii audience was probably turned off of it’s more “hardcore” based games, and more traditional controllers. (One reviewer even stated he felt Nintendo abandoning the Wii remote, even though it was fully compatible with the WiiU, and some of it’s games also hurt the system, as people were more used to it by now.)
So honestly, backwards compatibility is not the main problem in game consoles, in fact, it’d be more of a selling point.