I'm fairly certain Skyrim could've been done on Wii U; Assassin's Creed 3 and 4 got ported over and I'm pretty sure those are harder on the CPU than Skyrim. Same for Watch Dogs and Arkham City actually.
As for BOTW, it's a case of Nintendo knowing their own hardware better than anyone and pushing it to its limits.
Yeah, AC looks better than Skyrim and runs on WiiU. I'd say ACIV (taking art style into account) maybe looks even better than BotW.