As I'm sure you know, despite the District of Columbia's large population, D.C. residents lack meaningful representation in Congress, as well as full authority over city government. Writing for The New York Times op-ed page, Kate Masur explains why:
One problem is indifference; most Americans are unaware of the capital's anomalous status, the city's “Taxation Without Representation” license plates notwithstanding. A second is partisanship; to establish a vote in Congress for Washingtonians, who are overwhelmingly Democrats, Republicans would have to place a moral imperative ahead of partisan interests.
Another is race. A half-century after the dawn of the civil rights era, many Americans still have a hard time seeing African-Americans as citizens entitled to the rights that so many white people take for granted. For residents of a place once known as “Chocolate City,” these attitudes are a sadly familiar obstacle to equality.
I'd put race and partisanship at the top of that list; even with indifference from the public at large, if political gains from granting D.C. full voting rights were equally distributed then Republicans would be happy to support complete enfranchisement. Of course, this is only possible in a District where the voting strength of African Americans is weaker, and whites are a solid -- if not large -- majority. In other words, as D.C. moves toward a white majority -- or small black plurality -- the odds for complete enfranchisement approach 1.