Fire power. The Thompson had wide federal service with law enforcement, and oddly enough there were precious few choices for fire power at the time. Semi auto sporting rifles were uncommon, and the bolt action rifle was still the global military standard. A Thompson was the right blend of raw firepower, compact size, and (relative ) easy availability. (A Thompson retailed for the equivalent of about $2500 in today's money. )

Clyde Barrow favored a Browning Automatic Rifle (BAR) that he had stolen from a national guard armory.

Having machine guns gave the gangsters parity with law enforcement in a world still dominated by low capacity manually operated arms.