It's a cost thing at the end of the day. Activision will put Call of Duty on the Wii U and it will sell enough to make it worth the cost of money and time. Treyarch has experience porting the COD games to the Wii U anyways. With Destiny there's no guarantee it will sell well. Third party games don't do well on Nintendo home consoles. Bungie would have to put the time into porting it to Wii U or get someone else to.