Email is important because it's prevalent, simple, and based on documented open standards. It has an extremely high adoption rate, as close to 100% as you're ever going to get.
Do you really think grandma is going to install darknet software on her computer? Do you think an enterprise would embrace the software you're promoting as a realistic alternative to email?
Although it would be really hard to get people to agree to use PGP for email correspondence, this is a million times easier than trying to darknet the whole internet.
