Low Profiles
Even though they were on the most heavily guarded prison in the country, the guards paid less attention to a bunch of bank robbers. On the island there were serial killers and murderers; a few bank robbers were seen as petty criminals in comparison.
Rich In Resources
They had another advantage, and that was that the prison was actually a prison too. Many inmates were employed to work there, manufacturing furniture, clothes and shoes, and even certain things for the U.S. military. The island might have been remote, but it had its resources.