Because Bungie don't have the manpower to develop for that many consoles at once. They've gone from only making one version of a game, to making four different versions. That's also the reason why there won't be a PC version within the foreseeable future. Although Bungie employees has been slightly optimistic about a PC version in interviews, and said they just don't have the time. So we might get a PC port later. I don't believe anyone's asked them about a Wii U version, though.