As Carbon has incredibly long late-reg's, I can only suspect they want to make sure the buy-ins equal or surpass the guaranteed payouts so they are not out of pocket.
That being said, they do tend to be far too long. Yet, if someone wants to buy into a game late and start with fewer than 10 bigs, good luck. And near the end of late reg, that could very well be the case - or worse. I've been in some smaller games where it lasted until those buying in had about 4-5 bigs. You wouldn't think anyone would still join, but here we are.
Not surprising, really, as many will rebuy ten times for 1.10 into a game for a 5.00 ticket! If they even make that!