Despite the "port authority" rules on 'street pricing', the real reason is lack of competition.
A single vendor receives the food contract for the airport, and they now have monopoly positioning and a captive audience. When business X is the only seller, and when the customers are held captive and unable to "go elsewhere" [1] then prices will naturally rise to the maximum the captive audience is willing to pay.
[1] How many air travelers are willing to exit the security perimeter, to then need to take a cab to somewhere (most airports are not located near dense shopping/restaurant areas) to purchase food, to then have to go back through security to return to their flight? And what few even have enough time between flight legs to even consider that "go outside the airport for food" trip as even possible? Plus by the time the "cab fee" is factored in, even if they could find the identical sandwich for 5.50 on the outside, the $10 + tip or more cab fee there and back would make the sandwich $15 or more in the end anyway.