Menus have to be limited
There's just not enough space to have a ton of options. Or room to store the ingredients.
People expect their food quicker
There definitely isn't table service, and there might not even be tables. Odds are people are either on the way back to their office or have been out at bars, so the normal 10-15 minutes they'd expect to wait for their food is condensed to less than five.
Tight quarters make prepping awful
Moving around in a normal restaurant kitchen is already an intimate dance, but in a trailer you're essentially grinding on each other. And not in the sexy, R. Kelly way, though he probably should write a song about getting down in a food truck. Or maybe he already has...