Why am I stuck with Coserv as my electric and gas provider in Prosper, TX?

We recently bought a house in Prosper, TX. Prosper is on the far north side of Dallas, up by Frisco and McKinney. While trying to set up utility service for the new house, I've discovered that Prosper is a non-deregulated area - which means no choice of providers! You are stuck with Coserv. What is this about? What was the point of deregulation if some areas can simply choose not to deregulate and continue to gouge customers? Why is the "master planned city of Prosper" tolerating this? Anybody know?