For the worlds that chronically have a high number of openings from season to season, you have to ask a couple of questions:
1) Why do they have so many openings?
2) What is the commissioners of these worlds doing to try to reduce the annual turnover, i.e. attract owners who will tend to stick around and not be "one and done's"?
3) How active is the commish with respect to recruiting?
It's been said before by others, and I'll reiterate it again: good worlds have no problems filling. They generally have low turnover, they have very active commissioners who do a good job in recruiting committed new owners, and they sometimes have a waiting list of owners wanting to join. Bad worlds tend to have high turnover, commissioners who are there in name only but don't really do anything, and they take forever to fill.