It's different in each situation, but often these are locations where there are very limited catering services. In other words, there might only be one caterer available to us, and they're not used to working with airlines as we would see in a more normal civilian airport where the costs could be lower because they are providing greater quantities.
The location is selected by GAC in coordination with the host country, of course, for those reasons, which include security. Ultimately, there's often less choice when it comes to the actual caterers that are available to us.