Reason being it's a semi-private beta, not public release. Invite-only lets them ease into managing a larger userbase and the associated content.

It might also be because people tend to be more interested in joining a site if the barrier to entry is higher. It comes across as like that exclusive nightclub where only the 'coolest' people manage to get in.

Do that, and your community can get a lot of attention due to all the people desperately asking how to become a member of it.

Probably makes it easier to grow a nicer, more positive culture.