Each MLB team has territorial rights (it varies, based on some grandfathered or waived by the ML/MiLB clubs, but it's supposed to be 50* miles), and Angelos said no to a franchise in Waldorf (which the Nats were interested in, apparently).
Edit: Rule 52 clarifies that a MLB club can block any ML or MiLB club from playing within 15 miles of its territory without permission.