ding ding ding
That's one major reason, but along with that, comes the extra coding requirements. They've been away from GC for a while now while they've been pushing PS2 to its limits (and bringing XBOX along for the ride). I'm sure they are applying a lot of the knowledge gained from BO3 and Revenge to the creation of Black...and since none of that experience exists on the GC hardware, I don't think it would be a simple port. They'd end up putting a lot of work into the GC version and likely get nothing out of it...