Few people owned a Wii U. Porting it's games to Switch allows those people who didn't own one to enjoy those games. Simple really. One thing that really REALLY sucks about Saturn and Dreamcast is there is hundreds of games that never came to anything else and it's a shame as there is some real great stuff most people missed.
Switch still needs Tokyo Mirage Sessions #FE,Xenoblade Chronicles X. I think it's a given at this point Bayonetta 1+2 & Wonderful 101 are coming to it.
I have a Wii U but no Switch btw.