I don't know, but I think the writers of the constitution feared that representatives of a state containing the capital would have unfair influence over the federal government. When the capital was in Philadelphia, I don't know if Pennsylvania delegates at the Continental Congress exploited their position in any way. Maybe the concern was theoretical, maybe not.

>Why was the district of columbia created?
>
>>I may as well mention here that the people of the District of Columbia do pay federal income tax and can be drafted and still have a non-voting delegate in congress. That's why their license plates say "Taxation without representation." That tag was removed from the presidential limousine.
>>
>>They do get to vote for president.