Most developers have stopped making games for the Xbox 360 and PS3, notice most of the big games this year don’t have a last gen version, or stuff like Black Ops 3 which only has multiplayer and looks pretty bad. Reason that anyone would make a game for last gen and not Wii U is because between the PS3 and Xbox 360 there are 160 million consoles out there, vs. about 10 million for the Wii U. Also, many studios already have tools developed for Xbox 360 and PS3 along with years of experience developing for those consoles and don’t have any experience with the Wii U, which means it’s more work to get a game working on the Wii U than the other consoles.
Also, UE4 does not support Xbox 360, it does support Xbox One which is much more powerful than the Wii U.