Though I agree measuring only "international" tourists is a really silly way of factoring. As if driving one hour across the King Fahd Causeway... or taking the train from Budapest to Prague where there aren't even any international borders or passport controls... is somehow more significant than a family from Shenzhen making the trek to Beijing, or someone from Vancouver traveling to Ottawa. It reveals a very distorted picture of the number of tourists some of these cities receive looking at it this way. The entire country of Bahrain is smaller than the city of Riyadh and Saudis do not need a passport to go there.
There's no country close enough to DC to contribute large number of visitors, and it's not that popular for overseas visitors. So many people just stay in NYC.