Email's a finicky form of communication. It feels like casual personal correspondence when you're typing it but there's a prevalent school of though that all emails, despite the silly disclaimers we attach to them, are official company records.
So, hypothetically, when Billy G sends an email to Stevie B that says "DOS isn't done til Lotus won't run" that's not just some guy having a crack with his mate, that's (potentially) evidence of malicious intent.
Email seems like private communicate but it often becomes discoverable public content. Whereas Twitter et al has no false expectation of privacy. When you know you're addressing the public you tend to mentally compose yourself properly.
I'm sure we all see this sort of thing all the time where someone sends a meticulously phrased 100+ word email describing a proposed plan or task or other work related activity and gets a response of "Ok" or "Yeh sounds good lets do it".
For many people, not using email is often a good thing. The less opportunity they have to stuff up, the better off they are.