Email is the "dumb pipe" version of communication technology, which is why it remains popular. The idea behind email is that it's an unmediated communications medium. You send a message to someone. They get the message.
When people send you messages, they stack up in your in-box in reverse-chronological order, with the most recent ones on top.
Compare this with, say, FacebookFacebook, where you post a status update to your friends, and some tiny minority of them get it. Or, you send a message to someone on Facebook and the social network drops it into their "Other" folder, which hardly anyone ever checks. Alles zu Facebook auf CIO.de
Of course, email isn't entirely unmediated. Spammers ruined that. We rely on Google's "mediation" in determining what's spam and what isn't.
But still, at its core, email is by its very nature an unmediated communications medium, a "dumb pipe." And that's why people like email.