To be honest, although a lot of people still own the X360, Avalanche would want to make the best game that they can and to do that they will need to fully utilise the game's engine for Next Gen, which would be hindered by current gen.
Trouble is that different consoles have different coding, so making different game versions is not as easy as automatically converting file types. A lot of files can probably be converted, but many of them have to be completely remade. The point is that it's a financial decision. They don't want to do lots of extra work that might not be worth it.
Maybe they are considering it, but they've said that the game would be "perfect for next gen".