For one thing, snap chat is audio visual, email is not. The second is snap chat is designed to work in manner to capture the moment without interfering with it, email on the other hand has a very interrupting flow.
So you end up with one product that captures the emotional state of a moment, and the other which doesn't. This weekend for example I received a snap chat taken a friend bombing a motorway exit on roller blades. The fact it self destructs adds to this as well, because its not the content that matters so much as the vibe of the message. When I got that message I was stunned and wanted to see it again, I had the impluse to contact my friend immediately and respond. I would never get that from an email.
I probably get in excess of 100:1 snapchats to emails from friends for those reasons. I also get maybe 20:1 vs Facebook messages now as well now.