Edit: Hah, I get to eat my words. Turns out USDA and APHIS have been trying to fly planes over Mexico to release the sterile flies, but the Mexican government has been restricting the flight days and denying landing permission, which has hampered the program. Looks like the Mexican bureaucracy is the one failing here, and USDA/APHIS might be running pre-emptive releases in the US (but I can't find a source on that). They just agreed to lift those restrictions and cooperate more at the end of April.