Of course there are lots of reasons for this:

Usually the truck operator is the owner, so there’s pride of place; owners keep clean a lot better, usually, than employees who are not vested in the success of the business. Second, of course, it’s a smaller area to keep clean, and it can be as visible or more visible than the kitchen in a restaurant. Third, obviously, roaches, mice, rats etc. are not nearly as common in a mobile vehicle as they are in a restaurant with a nasty alley full of dumpsters and trash. So – those of you who are scared of taco trucks, there’s really no reason to be. I find that some of the best Mexican food I’ve had in California is from these trucks, from tiny farm communities in the central valley to those serving folks outside of clubs. Here in Sacramento we’re lucky enough to have three great trucks in one particular neighborhood, and they often have lines of people – Latin, white, African-American, Asian – outside every night until midnight, they’re that good!