Kinda basic but takes time. Time to merge the bug fixes that have piled up on the live build since the last split. To merge them with the main trunk that carries the new features added since the last split. Time to reconcile the fixes with main code that's diverged, and catch and fix the bugs that this itself introduces. Time that's more than it should be because just operating the game is hampered by the backlog of unfixed bugs remaining because you didn't get to complete the above on the previous few releases.
Time you don't have until the main trunk is locked down. Meaning if the main trunk feature development slips schedule, even with late cuts to try to recover the schedule (carriers, ice planets), when you are facing a looming December release deadline, it is TIME YOU DON'T HAVE. You are stuffed. Management has to choose what to release -- the bug-fixed build without new features OR the bugged build with new features. That or delay the release, miss the bi-annual financial report deadline, and take the hit.
Go figure, as they say. I truly admire the ED devs who have stuck at their jobs through that for the past three years.