I think it has to do with the lack of knowledge of the Wii U platform. Is it possible? Most likely, but it involves investing money and hiring people who know the platform. Not to mention Destiny is a game that focuses on online multiplayer/co-op. Nintendo's online is not the most reliable thing in the world. And this is coming from a big Nintendo fan... If the game isn't even coming out on PC initially, my guess is a Wii U version is not very likely to happen.