These shows are still a business. I'm sure there is a chain of command, securing funds, making sure all the actors dont have previous commitments, etc. Likely very hard to lock everyone down at once, especially actors that do both TV and movies.
I thought it was a solid premiere. I get the appeal of people wanting it to be like BB, but it's good in its own right. I've said it before, and I always get shit, but I'll say it again. In many ways this show is superior to BB.