Sonic Mania is quite possibly the most popular Sonic game that the gaming world has seen a very long time.  Considering the fact that the Wii-U was discontinued earlier this year it would not have made to much sense to have Sonic Mania ported for that console at all.  In fact Sonic Mania is one of the many titles that would make people consider buying the Nintendo Switch.

The  Nintendo Switch can also be utilized as a handheld console which makes it stand out from the Nintendo Wii. Since the Wii-U paled in comparison to sales for the PS4 and the Xbox One having Sonic Mania on the Wii-U would not have been as big as having it on the Nintendo Switch.