Because they'd be killing their future ability to make some of these games. For example, each console only gets one Mario Kart. There might not be a MarioKart 9 on Switch thanks to 8 Deluxe. If Smash for Wii U was ported, the Switch may not receive a new Smash game. I'd rather have them focus on making new games rather than catching everyone up to speed on all the games they missed with the Wii U. The Wii U is dead and done. It's time for SWITCH, and all the new games that come along with that. I get that they need some titles to fill in dry periods throughout the year, but I really hope in a year or so they can get to having a steady stream of fully original titles, not ports.