I stand corrected, though I do believe the restrictions exist to ensure that the National Mall continues looking majestic.
Partially true, it is one of the reasons, the other being that Thomas Jefferson wanted DC to be an American Paris with nice, airy streets and people like to take things he said to an extreme. I'm pretty sure if he saw the amount of sprawl in the area (and all of the problems that is causing) he would be okay with tall buildings.
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