That is a very good question. However I doubt they wanted to introduce 120V!!!! on the ramps or similar board. Once you have a heated bed on 120v it would be more complex in controling the heat. For example ABS from what im gathering needs between 50-65C to adhere well. Other plastics need less. I am aware you could put in a separate controller on the 120v heated bed to get the proper temp then its another step before printing, when its all in one, theoretically once you have your gcode generated you put it on your sd card and let her rip. Once the bed is hot enough it will start printing. 12v makes for a more user friendly environment vs the potential of killing someone who is trying to enter the world of electronics for the first time.