Boston and NYC are all developed expensive. Atlanta still has undeveloped area inside the city limits or old industrial areas that can be removed and replaced.
Chattanooga wouldn't work because they want an international airport and major university.
Atlanta has the world's busiest airport and Georgia Tech (engineering), Emory (business, medicine). Atlanta is also a huge transportation hub of the Southeast.
Detroit is a cesspool, but does have the added advantage of everything being cheap, lots of places to develop, and lots of transportation infrastructure, but no international airport.
It looks like Amazon wrote this RFP for Atlanta.