Because they want to appeal to a broader fan base then just Prince William County
I understand what you're saying, but I still think it's lame.
I recall the owner changing from "Prince William" to "Potomac" when he got ticked off that he couldn't get cooperation on building a new stadium, so changing the name to Potomac was part of his threat to leave the Woodbridge area. But that's not an issue anymore now that the stadium is going to be built in Woodbridge.
No big deal, I just think it's kinda lame. Sort of like calling the Denver Broncos the Rocky Mountain Broncos or something.