Thanks to all who replied. At Bob Gardner's suggestion, I emailed a
gentleman at Potomac who took the time to look into what happened and
to let me know the full story. It all makes sense, even if it was
puzzling at the time. As an asside, it's great that people in ATC take
trouble like this to let us pilots know what's going on under the hood.
It's part of what makes the US ATC system such a pleasure.

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Mike,

I think I have an answer for you. I talked to someone who specifically
remembers the incident. At the time the Center called, the controller
at Potomac was very busy accepting deviations that were landing at
Dulles. There was weather that was affecting the main arrival route
into Dulles (the one that comes over V143 and then over FDK). Aircraft
were deviating in that area. Additionally, another aircraft which was
pretty much flying the route that you wanted was deviating about 20
miles south of EMI for weather and could not get back to the north
trying to get to HAR. I think he departed JYO. The controller just
could not handle your flight at the time. I am glad it worked out for
you, getting to THV by way of Scape. Sorry we couldn't have been more
accomodating. If you have any other questions in the future, please
don't hesitate to ask. Thanks.

Scott Proudfoot
NATCA Eastern Regional Safety Rep
PCT TRACON