Correct me if I'm wrong but if you want to send an encrypted mail to person A, your have to use A's pubkey. How do you encrypt all your mail if not all of your correspondent have setup their pubkey/privkey GPG system?
It would be like calling someone who doesn't have a telephone. The solution? Encourage or force them to get one, or buy one for them and show them how to set it up.
And I'm sure that's too much of a hassle for everyone to do. That's why a service like Lavabit was perfect to the core.