At West Georgia, NPC sororities are in 1 dorm - AGD,Chi O and Alpha Xi on one side and KD,Phi and DDD on the other side. We've had the sorority dorm since around 1972.
Every year, there is a dicussion about wanting to move off into a house or get a whole apt. building. Unfortunately, I think if 1 moves, everyone has to. Last spring, the college adopted a policy that each sorority has to be financially resp. for 25 (?) beds. They announced the reg. Spring Qtr, after several people had signed apt. complexes. To make it more difficult, freshman girls can not move onto the sorority dorm. (I don't know if there is a rule that freshmen guys can't live in off-campus fraternity houses.)
It can be a financial strain if a sorority doesn't have 25 sisters who can/will move on the hall. I think that a sorority has the option of letting non-Greeks live on their hall.
A house can be a wonderful thing, but you also have the liability pressures that a dorm room doesn't.