At the inception of the government, the thought was that senators, representatives, the president and their staffs would not be residents of the District of Columbia, instead they would be residents of their home states, and thus they have voting representation on congress thru their home states' representation.
The founders did not foresee the growth of the population of DC, (or for that matter the federal goverment) and did not see the need for DC to have independant representation in Congress.
The very reason that DC exists as a seperate entity from any state is so that any single state would not have undue influence over the federal government.
Founders thought that Maryland would be able to influence federal officials to Maryland's stand on an issue by passing some ordinance (for instance) prohibiting the breed of dog the Massachusetts Senator favored, and he would change his stand on an issue to allow Fido to come to Washington.
Amendments have been proposed to alleviate this, and while voting congressional representation is still not available to the citizens of DC, they do have a non voting representative in the House, and delegates to the electoral college and therefore have a vote for the president.
