“The park was conceived of as part of “a necklace of emeralds,” large permanent reserves of open green space connected by trails that would ring the city. “ -from the McMillan Sand Filtration Site Wikipedia entry.
For the longest time, I was convinced that the CIA had a secret facility under the McMillan Reservoir. Why else would a huge tract of well-positioned land remain undeveloped in DC?
Like with most conspiracy theories, there turned out to be a perfectly plausible reason: money and politics.
Bloomingdale resident Eva Hambach created a documentary piece about the site development, available on YouTube.