The economy of scale. If we could get cars to run on it, it would be $.75 a gallon. (maybe a better avenue for car companies to look into than hybrids). Still, people use billions of gallons of gas a year. JP-8, not so much. Thus you have to charge more per unit to cover production costs. At least thats my take on it.
FQ13
While I agree with your reasoning, I don't completely buy your argument. While I don't have the numbers, we are always bashing the environmentalists for flying around in there private jets using more fuel to go from L.A. to Washington than it would take to run a HUMVEE for an entire year. Given how many flights there are on any given day, (both private and commercial) and lets not forget the other vehicles that run on JP-8. (helicopters, some types of boats, the Abrams tank) I don't think supply and demand is a determining factor in the cost. I am leaning more towards Timothy's reasoning that it is probably taxed more, probably because it is something used mostly by commercial entities, and not so much at a general consumer level.