And the ones that are left aren't especially interested in paying for copies sold. It's easy to stay enthusiastic about an expanding, growing phenomena. And it's easy to burn out on a dying one. As fun as it may be to put out a publication for 50 like-minded lunatics, sooner or later, it does get old. People and passions change.
Stagnation sets in.
Suddenly, getting that new issue out becomes less and less of a priority. If you're not growing, you must be dying. Many of the best zines may be dead, but the editors live on. Odds are, your favorite zine editor has moved on to greener pastures, most likely freelance writing. You can call it selling out.