"Audience" or user base demographics more than anything are determining Wii U developer support.
Granted, it's still a bit early to do a legit breakdown of the Wii U user base despite coming up on its second year of availability, because sales simply haven't taken off in any significant manner. But, I'm inclined to believe that the Wii U has probably replaced more Wiis than XB360s or PS3s and that the heart of the Wii U early adopters are the Nintendo core audience.
About the only consumers I can think of who would choose to buy the Wii U version of a multi platform game over any other would be those who only own the Wii U, or those who just bought one and understandably want to play something on their brand new console.
The only problem with the latter is that those who bought a Wii U early, probably have sights on a new XB1 or PS4 which means those multi platform games will be purchased for those systems instead.
As for the online play, the only thing that really hindered online play for the Wii were the friend codes and the general background nature of network functions which have changed with the Wii U. I won't even comment on network infrastructure where the problems are usually related to more traffic than the network can handle which is one problem the Wii U does not have.
Hardware is a bit of a given and it will plague the Wii U from the standpoint that straightforward ports will not be so straightforward due to the fairly significant difference in memory and processing resources.
Easy ports are going to be out for more resource intensive games and when paired with the Nintendo audience factor, said ports are a much harder sell, particularly when compounded by the small user base that I don't see magically changing for the better after the PS4 and XB1 debut.