“There’s a cost issue. Technically, it is possible for the Wii U to support multiple Wii U Controllers. We’re not planning on asking our customers to buy multiple Wii U Controllers.” – Satoru Iwata

This makes sense but I really hope they can get at least 2 bundled with the console or some way of making a slightly higher priced bundle with 2.  And then for 4-player games, you could have friends bring their Wii U controllers.