Because they want to appeal to a broader fan base then just Prince William County
Brief History Off the top of my head. Forget when they started using Potomac instead of Prince William (PW)
Alexandria Dukes - Seattle/Pirates
Prince William - Pirates - Pittsburgh - Bonds
PW - Yankees - NY - Bernie Williams, Jorge Posada,
PW - Cannons - NY Yankees/Chicago White Sox - Magglio Ordonez
PW - Cannons - St. Louis Cards -
Potomac - Cannons - St. Louis - Rick Ankiel (Pujols may have come thru here but not for long)
Potomac - Cannons - Reds
Potomac - Nationals
I'd have to check the Potomac Nationals Website to research the teams. I may have St. Louis and Cincinnati turned around. Not sure.
Corrected with Sens 2005 comments