I love the wii u but the answer is fairly obvious.... programming for the Wii U is a very different experience from the PS4/Xbox1, including the need to make use of the gamepad. This costs extra time and resources, and they tend not to feel it's worth doing as the games usually sell rather poorly.
It also doesn't help that the last few big third party games were trash (looking at you, super-late-and-overrated-Watch_Dogs) that justifiably bombed, of course.
As for your "they can still make money releasing on the Wii U", a bit of profit alone does not make the effort worth it. Look at it this way: they could make some money porting a game to the Wii U, but the time and resourcse they dedicated to making that port could go towards an effort that would net them a lot MORE profit.
