Although I do not know why it is successful but you are right; mail is definitely not the reason why it is popular. But I don't think it is for the reasons you think it is popular either. Of all my 25-early 40 year old friends working in enterprises, exactly 0 people like it and they avoid it like the plague when they can. They like it slightly more than Sharepoint, but that's not saying much. To why it is popular still I would wager that's because enterprises just 'have it' and you have no other choice; you are forced to use it. But you were probably going to say 'collaboration' ?