Both: Airlines from other countries refuse to fly in US airspace because of dangerous ATC and planes from the US are refused entrance in the airspace of other countries because the FAA isn't trusted to enforce proper maintenance schedules on planes from US companies and other countries don't want the additional cases of planes falling from the sky or crashing on landing that would result from de facto unregulated air travel safety in the US.

Also Boeing sales would suffer if the FAA was officially deemed in other countries to not have equivalence with their local air-travel regulators when it came to oversight of plane manufacturing because it wasn't actually doing the job of preserving the safety of air-travel.