Upvote:4
He lived after Christianity became legal; whereas many of the writings of his predecessors were burned in state-sponsored persecution (source) his were much less targeted for destruction.
He lived in Constantinople, which would serve as a bastion/protector of Greek Christian writings until it fell in the 15th century. This meant that his writings stayed in circulation long enough to be copied many, many, many times. See further discussion on the Great Library of Constantinople here.
Contrast the relative safety (at the time) of Constantinople with Rome (conquered multiple times in the following century) or Carthage, which was devastated by siege and captured by Vandals in 439 (source).
(Chrysostom was also just a very prolific writer)