Great question — answering as the UphillTech team:
There was never an explanation in the show as far I can remember, but the creater (J.G. Quintel) of the regular show had already played around with the idea of a humanoid gumball machine before. In his student film 2 in the AM PM there are two characters that take drug laced candy which turns them into Benson and Mordecai, characters we know from Regular Show. I don't think there is currently a reason as to why Benson is a gumball machine, it's just the cartoon world, everything is possible. It's more likely that Quintel is paying homage to his earlier work, but until he speaks out about this, we won't know for sure.
Full answer by JustMike: https://movies.stackexchange.com/a/61193 (CC BY-SA 3.0).
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