The machines are way undervalued, so people aren't selling them. When they do pop up, more often the machine is really beat up, like the mylar is peeling and inserts losing the paint, etc. So unless you're lucky and get a nice one, you end up buying a project to restore.
It seems like the only place I reliably find older machines for sale is at the expo/explosion. There was a Diner there last year, and since then not once have I seen one up for sale anywhere.