USNS ships are crewed mainly (80%+) by civilian mariners. The US Navy does not recruit civilian mariners for enlistment like with USN ships. They hire them from the wider civilian workforce.
The available workforce is too small due to a variety of reasons but the main reason is that $65k-250k isn't enough for a physically strenuous job with strict licensing requirements where you're away from your family floating out in the middle of the Indian Ocean with no port calls for half the year, minimum.
The US Navy, and therefore the Military Sealift Command, has an operational tempo that keeps increasing and qualified mariners generally do not have trouble finding employment with better working conditions.