*Membership spots not really limited!
For Cassandra's case see my OP/ED. There's some evidence to point you in the directions of the numerous ways DC tried to get rid of her in the past.
Stephanie? I think the higher up's never liked the character to begin with. However, Didio saw the appeal of her as Batgirl and went with it before the higher up's probably demanded he try the original idea proposed with Barbara back as Batgirl.
Yeah, it's been obvious to me for a while that editors outright hate any Batgirl that isn't Barbara Gordon. Cassandra's turn to evil was badly written, and Beechen didn't do his research, but it was an editorial mandate. And I know that he was trying to redeem himself with her miniseries, but I doubt that's why editors gave him the job. Seriously, though, someone should remind DC's editors that they aren't writers.