I have a theory as to why. I think it has something to do with generation continuity. Destiny will likely be the last game available on both current gen and next gen platforms. Destiny's sequel will likely only be on ps4/one/PC. Bungie would not longer be limited by current gen platforms and would be free to fully exercise next gen console's capability. We can assume (based on common logic and leaked material) that a sequel will come in no shorter than 2 years. We'll be well into the next gen by there and people who now play on current gen will be able to pick up on the respective next generation versions of the console they play on. Assuming Destiny game out on the Wii U, it's sequel would be expected to be supported by Nintendo's console too. But the Wii U is just about as powerful as currrent gen consoles and isnt even comparable. Bungie doesnt want to be weighed down by current gen anymore. No one does. By the time they are ready for their sequel, there will likely be no Wii U successor and they would be expected to support those who played Destiny on the Wii U. There'd be something of a rift (all assuming they dont chose to hire a separate team to create a Wii U compatible Destiny 2 and leaving other consoles unaffected.