The risk of not being able to buy back in to the market you're selling in is too much for many people. The normal, and healthy, movement of buyers/sellers to a smaller/bigger home isn't happening.
It's like a game of musical chairs, but when the music starts the players won't get up. They just stay seated and grip their chairs.
For the most part, the people leaving the area are the ones listing for sale, and there aren't enough of them.