Because running a mail server can be a pretty big responsibility, especially if you're letting others send mail from it. The more people using your server, the more important it will be to monitor for abuse and deal with abuse reports.
When I first set up my mail server I was pretty excited about being able to help everyone I know get their email away from Google, Hotmail etc. But once I had it running, I quickly realized that I didn't really want to give to very many people. Even if I trust all of my friends not to abuse, I cannot trust all of their computers.
I was using mod_gnutls for some sites, but it did not make it into Debian Jessie. mod_nss is still in Debian Stable though.