“Does Congress not administer D.C.?”
Nope. D.C. got home rule in 1973. Thus able to elect POSes like Marion Berry.
Article One, Section Eight of the United States Constitution grants the U.S. Congress "exclusive jurisdiction" over the city. The District did not have an elected municipal government until the passage of the 1973 Home Rule Act. The Act devolved certain Congressional powers to a local government administered by an elected mayor and a thirteen-member Council of the District of Columbia. However, Congress retains the right to review and overturn laws created by the council and intervene in local affairs.
Thus, unless the Constitution is amended, Congress has ultimate say. Of course we need a Congress with the stones to do it.