Because after very little sleep, when I have to be up and out the door early with 2 very young kids and a teenager, I can hold everything and pop a can in a pocket.
It's cold so I don't need to worry about burning anyone.
I can have it almost frozen, so it really hits the spot (I love ice...think its linked to anaemia I can't get rid of)
Its just convenient, I don't really like the taste though, just accessible caffeine throughout the day.