Once upon a time, in a land far, far a way. There was a taco vendor that sold tacos out of a tool box......
No seriously, I don't know exactly for sure why it is called that. I have a feeling though that it is based on the similarity to a taco wagon. Between the drawer layout and the vertical folding panel on the back side. When that box came about bottom only boxes were not as prevalent as they are today. It was either that, a "Roll a bench" or a standard top and bottom combo. Today's boxes kinda follow the taco wagon mentality, bottom only box with a top work surface.