A lot of it is relationships. A lot of it is the kind of workforce you'll get (people already experience with DoD stuff and have the right clearances but don't want to move out of the DC/MD/VA area), and such things.
Amazon's big draw for someone currently doing cleared work is being able to ditch the clearance without taking a pay cut or moving. The people who want to work for Amazon and keep the clearance have already made that move.
