Well, list the alternatives:
- NNTP-transports (though not necessarily part of the Great Usenet Cloud)
- Web-boards (such as, but not necessarily like, perlmonks)
- Phone calls
- In-person meetings
We can toss out the last two easily enough. Having solid, permanent records of either is a pain, and actually participating in either is also very limiting. #4 is geographically limiting, and #3 is financially limiting. (Conference calls are expensive to set up, and expensive for many users to participate in, so they're out.
#2, web boards, are more accessible, but they suffer from a requirement to be online to participate, and actually doing so requires dealing with the godawful user-torturing editing widgets that the browsers inflict on people. Yes, I know, some Clever People use Emacs as a browser, but 99.9% of the folks are stuck with a Netscape, Mozilla, or IE variant, and they all suck.
#1, news, generally requires you to be online as well to participate, though at least most newsreaders have decent editors, and let you use an external editor if they don't. Some (very few) newsreaders let you work off-line, but they're quite rare.
Mail is really the only option that has the combination of good editors, off-line usage, decent web archiving, and NNTP gatewaying, giving the broadest group of people access to the discussion. (A well-administered NNTP system, with a mail gateway, is about equivalent, though slightly more trouble to set up)