Majority of 3rd parties use UE to make their games, EA is using Frostbite mostly. If those 2 engines don't support Wii U, it'll be too much of a task for said 3rd parties to release Wii U games and they will be much more likely to not bother releasing games for it.
Unreal is popular, but I don't think it has more than a plurarily. It's also not unreasonable to think that Unreal 3 might persist into the PS4/Durango era longer than Unreal 2.5 did for the HD twins, simply because it's a more capable, scalable engine that's already confirmed to be used on some next-gen/cross-gen titles. (Star Wars 1313, Thief 4) Being left behind with Unreal 3 is bad, but not nearly as bad as the Wii's predicament of having to recycle PS2/PSP games.