This one comes from Felix Salmon. In my view Felix puts forward the two correct hypotheses:
…food trucks are much more likely to be run by first-generation immigrants, for a variety of reasons. Quite aside from any hard-working immigrant stereotype, that’s good news just because the food they sell is going to be that much more authentic. (Not that food trucks need to be particularly authentic to be delicious: just ask the Korean taco people.)
My favorite theory is that it basically comes down to the amount of time that elapses between the taco being made and the taco being eaten. Fillings can stay warm and delicious for a while, but the tortilla really is at its very best within seconds of coming off the stove, rather than getting soggy at the bottom of a tortilla warmer brought to you by your server. I suspect that if you could walk into the kitchen of a decent taco restaurant and get the chef to make you one then and there, it too would taste better than the same taco ordered off the menu.
I would add one factor. Taco trucks are mobile, and they often serve Latino construction workers, who are themselves mobile in terms of choosing various workplaces over the course of a year, and thus they require mobile sources of food. This encourages the taco truck, but not the stationary restaurant, to invest in better and more authentic food.