Because Washington D.C. is not a state they do not have any representation in Congress, but they still pay the sixth highest individual income tax rate in the U.S. This is somewhat ironic considering that the home of the U.S. government suffers from "taxation without representation," the very issue that led to the founding of the country.
But on November 8th a ballot referendum will determine if most of D.C. will be allowed to gain statehood. Is it possible that Washington D.C. could become the 51st state?
Tax Foundation: The Facts on the District of Columbia's Tax Climate
The Washington Post: Support for D.C. statehood at record high
Slate: How Powerful Is Your Vote?