In addition to some of the things I’ve mentioned, when evaluating applicants, the selection committee looks at past experience, skill set, and the quality of their work. The make-up of the existing group is also a factor; they might want someone with skill set that’s not as represented, for example.
And there are no term limits. If you’re a member in good standing (which includes attendance) you can return. Members also often take a season off when they’re deep in production, then come back. Folks sometimes move to a different chapter too.