A lot of past Nintendo consoles have made pretty major changes to the hardware and operating system compared to other console manufacturers, which makes backwards compatibility less likely. The Wii U basically booted into the Wii operating system to play those games. I imagine getting Wii U games running on the Switch wouldn't be the easiest thing to do, though certainly not impossible. We don't know a ton about what changes the Switch successor will make, so it wasn't outside the realm of possibility that it would be the same.