There are more important places to target (in my opinion) like military bases and even food storage or farms or what ever (destroy the food, the country dies). There might also be defenses against nukes since it is the Capital and all. But the White House is completely nuked, might had been enough.
During the cold war the primary targets were always nuclear forces, conventional forces, and population centers in that order. So really Forts Bannister and Constantine should be glowing craters, what with their ICBM silos. Most likely the area was well defended by interceptor missiles or whatever passes for ABM weapons in Fallout, and built extra durable just in case. Also, the warheads in use were supposedly lower-yield than their real-world counterparts. Short answer - radioactive craters make poor gaming environments.