Doesn't seem to be a printing issue, as the work has already been reprinted recently. A legal issue is unlikely due to the same reason, though it's still possible.
I'm gonna guess demand. It's an expensive ($99) item, that isn't going to have high demand. The issues were collected a few years ago. Any CB fan might have already got the collections and may not be willing to double dip. Of course Panni recently printed these collections in Europe as well.
Then you have the fact that there really is not "future" for the character right now with his series being cancelled. It's not like Marvel can say "hey, you liked the CB omnibus?, then pick up CB's current series". Compare that to the other omnibuses, which have more of a "future".
Just a guess looking all the factor involved. Collections are often delayed in order to have time to increase their orders.
...because if there is demand for Secret Wars I and II omnibi, then they shouldn't have a problem with Captain Britain.
I would love to see the actual sales on those Secret Wars volumes...baffling.