It would be A: annoying to have to pay another 60 dollars for the SAME game and B crushing for the switch to just have another port of a classic and well-loved franchise ( with the other, off the top of my head, being mk8 Delux
While I can see some people not wanting to double dip for a port if they already owned the Wii U version, you still have to consider that the latter is designed for a console whose sales are pretty low by Nintendo's standards. In other words, not a lot of people actually got to experience Super Smash Bros. for Wii U.
This gives its developers a perfectly good excuse to port the game to Switch. The latest console already exceeded the lifetime sales of Wii U with only 9 months, and it doesn't seem to show any signs of stopping at the time I was writing this reply. Considering that the development of Super Smash Bros. for 3DS and Wii U was incredibly taxing with its sheer amount of content to begin with, it's very tempting to simply bring the Wii U version and enhance it for the Switch.