Clustering is a very real thing, especially when it comes to the very best workers. We are starting to see that in the DC area: For quite some time now, there have been a ton of tech workers for government and government contractors, but lately non-government related firms are starting to appear on the landscape, attracted to the amount of talent employed in the area. If you're building a tech startup, your number one priority isn't land, (you don't need it) it isn't taxes, (you aren't making money yet) it is top personnel, and it becomes much easier to go to them than have them come to you.