Devs really don't have a problem with 3 systems (heck they've been porting to four recently when you include the Xbox 360 and PS3) but it's too costly when the porting process is as difficult as it is for the Wii U, from figuring out how to work with their strange architecture to having to come up with some use for the game pad while scaling down their game so that it runs smoothly on the underpowered hardware... If the Wii U was selling as well as the Wii did you can be sure they'd be developing for it but it's just not worth doing now.
Three has never been too much for devs (they were working with three just fine last gen), it's just about money. People didn't develop for the turbo grafx because no one in the west had a turbo grafx. If the Wii U was a similar build to the Xbox 1 and PS4 and was easy to port to, they'd be porting to it right now. As things stand, though, the process is too difficult and the install base too small to justify it.
