There are many arguably unjust aspects about American society. Some of them are quite hotly debated in the public arena, but others seem to maintain themselves more quietly. For example, I consider the denial of marriage equality to gay and lesbian citizens to be a grave injustice, but it certainly is not absent from our public deliberations about what it means to live in a just America. However, other topics which seem to equally implicate our fundamental values, such as felon disenfranchisement or the status of American colonial possessions (e.g., Puerto Rico, Guam, the Northern Marianas Islands, and yes, the District of Columbia), manage to exist without being the subject of intense public debate.
I say this because, despite being of obvious moral and ethical import, I've yet to read a convincing justification for the current status of the District of Columbia. The last time the topic came up in Congress, the main argument deployed against it was constitutional, not moral. It's not that I don't think constitutionality matters, but it does raise the question -- why not amend the constitution? The point is, there's really no argument that the status quo is radically undemocratic vis-a-vis D.C. (and Guam and American Samoa and the Marianas and all our other colonial possessions). But this is not considered to be a "problem". At least with gay marriage, which I also think is a rather clear case of definitive injustice, the topic is recognized as being within the realm of politics and debate. But nobody is grilling our presidential candidates about whether they support Puerto Rican representation. By and large, even civic-minded voters don't have this issue at the front of their minds. There is no groundswell of outrage that we're depriving millions of American citizens of their voting rights for no discernible reason.