One more thing to add . . .
Discovered this article today as well:
https://futurism.com/amazon-may-be-u...-day-shipping/
There is talk about building a hyperloop between NYC and Washington, with a <30 minute delivery time for passengers and cargo. Amazon has repeatedly stated they want to be on the forefront of technology and condense delivery times. Same-day delivery is a game-changer. That corridor also is highly densified with people, and both cities have relatively higher earning incomes that most places in the U.S.
There are multiple non-stop flights from Seattle to Washington, D.C. (and back). Germany and the UK are also the 2nd and 3rd largest markets for Amazon outside of the United States, generated over $20 billion in sales in 2015 combined. There are direct flights to both countries as well from D.C.
Cushman and Wakefield also rated D.C. as the third best place for tech, just behind Silicon Valley and San Francisco, with close proximity to over 150 tech companies - including Amazon Web Services facilities already there - and AWS is also the biggest source of the company's profits. There is a highly educated and concentrated skilled labor market, and relatively easy access for those out-of-state too.
This is in addition to Jeff Bezos buying a massive $23 million home in the D.C. area this year.