It’s that Washington, DC isn’t meant to have representation in Congress. Sorry, guys. I think if it was meant to, it would have been specified in the US Constitution, and what’s in the Constitution about DC (and the representation thereof) is too wobbly to be a good legal argument for that representation. Naturally, I allow that lots of people will disagree with me on this one. But fundamentally I think DC is supposed to be a funky federal fiefdom, a place where Congress can do its thing without aggravating one of its own members. It’s a bit of an atavistic view, I’m sure, but there you have it.
It’s one of the reasons, actually, that when I lived in the DC area I never considered living in the District; I like having representation in Congress.