We're not talking about some random piece of the United States, DC has a special existence.
But that's the point - DC doesn't need to have a special existence. It deserves to be a state just like any other.
What about individual state capitol cities, then? Do you believe they are special? If not, then neither is DC.
Like I said, Canada does just fine with having its capital in Ottawa, which is a normal Canadian city. Ottawa's not special, so DC shouldn't have to be either.