This is an internal Amazon decision that is not regulated in any way. They are not obliged to any notion of "fairness".

Amazon just put some cities on a list and negotiated with them.

Any city not on the list is not there because Amazon didn't want it there.

I also doubt that existing talent pool size was such a major consideration. Crystal City VA doesn't strike me as an obvious tech hub, nor does Newark. Amazon is definitely counting on talent to relocate to its new offices, especially if they put them in such locations.