I think it's mainly because the Wii U is getting into online-play really for the first time, and might not be able to run a lot of destiny's online elements(seamless public events) smoothly. I like the Wii U, and think it'll do fine with it's online play, but destiny REALLY seems like it's online-focused, where most Nintendo games were mostly meant for single-player or local play. It might just be a bar too high for the Wii U to reach right now. But don't worry, Nintendo will get there someday.