Au contraire. In all honesty, working in theatre/concerts professionally for the last 7 or so years and semi-professionally for the last decade, here on this forum in another thread was the first place I've ever heard comm referred to as cans.
I have, however, consistently heard headphones referred to as cans (and done so myself) many times in the last decade.
It all depends on who you work with and where you work. Terminology--especially slang--is different everywhere you go.
Oh well, that was the reasoning i got for it. People just probably call headphones "cans" because it is easier and faster to say that instead of headphones or earphones.