I don't think overproduction is a reason. I think that the finite number of available sellable licenses exist (and often these licenses spread over multiple manufacturers). There's not really an infinite number of figures you can make. For example when you produce numerous versions of Batman, Jack Sparrow, Spiderman, Ironman, etc, you're going to eventually run out of customers.
Plus not all licenses appeal to all collectors, so there is a limited number of customers. Like I hate superheroes, no interest at all. But I'd be all over another Godfather figure or something interesting that hasn't been done yet.
So that and price are the main reasons, in my opinion.
Sean