Weight, and the small size of the aircraft. F-16s are tiny. It would take a LOT more stored nitrogen gas to dilute the O2 level below the upper explosive limit, than it takes of CF3Br to chemically interfere with combustion. Modern aircraft use continuous nitrogen gas separators that run off engine bleed air to accomplish the task, at the cost of some weight and a ~3-5% increase in fuel burn. There was a study to replace the CF3Br with CF3I back in the 1980s but it was shelved due to technical challenges, which could have been overcome with additional investment.

https://www.nist.gov/system/files/documents/el/fire_research...