That was literally an Act of Congress. 49 USC 44506(f)(1)(C). (The general rule, at (f)(1)(B)(iii), is that they have to hire individuals "who apply under a vacancy announcement recruiting from all United States citizens.")
There's only one such metro statistical area in the entire US, and it's NYC. In order for the FAA to do local bids Congress would have to change the law.
I think they should, mind you, but there's your answer for the moment.
Learn something new everyday. Thanks.