How about because federal law and Bureau of Prisons allow defendants to voluntary surrender to the designated institution and judges commonly grant it to those who don't pose a flight risk and have been on bond throughout their case? It's a scoring factor on inmates' initial classification form which determines the security level of the facility they go to. Those granted voluntary surrenders get a three point advantage and can end up being designated to a lower security level prison. Geography has nothing to do with it.
It also costs a lot of money to transport an inmate from one area of the country to another on Con Air. (Justice Prisoner and Alien Transportation System (JPATS.) You don't get a direct flight, but usually end up in practical isolation in a transfer center like El Reno for a week. Here's the U.S. Marshal's directive.
The federal court may direct a prisoner to report, unescorted, by a specific date and time to an institution for the service of sentence. The BOP formally designates the institution for service of sentence. The USM is responsible for notifying BOP of a voluntary surrender commitment and for forwarding required commitment records to the receiving institutions. Authority for voluntary commitment is contained in 18 USC 3621 (b).
It's also discussed in 18 USC 3585.
This is an open thread, all topics welcome.