Initially it was because I run/ran the most popular Ruby blog so the first couple of thousand subscribers came almost exclusively that way - http://peterc.org/blog/2010/306-1120-subscribers-in-2-days-m... - but since then it has been primarily a case of word of mouth and "domino" growth.

For example, many Ruby developers are into Rails. And many of those are into JavaScript. So I branched out into a JavaScript newsletter too and had a few thousand subscribers quickly. Nice (and unsolicited!) words from people like Steve Souders and Paul Irish helped with testimonials and it grew from there.

The next domino was HTML5 which was of interest to both JavaScript and Ruby people. So that one has grown even more quickly. And so on.. :)