I would think a food truck would have a hard time keeping up with the quantity and need to be restocked quite often. They don't have the storage that can be set up in a tent. I looked at the food concession application, and Food Truck is one of the location selections, along with building or tent, so they are allowed.
Most of the food items I see listed on the food and concessions web page should work well with quick service, but others - especially from the area restaurants, may be too complicated. I'm guessing it's a management problem by not cooking to order during slower times.