MORE: (11th comment)
It doesn't seem to have much to do with being in the right place at the right time, as far as I can see. New craigslist local sites are added all the time and in very different places -- it appears that all you have to do is give it time and wait for the site to grow...
As long as you have no real competition. Most people are really frustrated at the point when they need help -- there is just nobody to help most of the users in need of help (way too many users per customer service employee and it's getting worse every day). But as long as they can't go elsewhere... they are stuck with craigslist, however badly treated!
In some foreign places, there are local options that have done quite well. As far as I'm aware none of these were non-profits. So given enough time, they couldn't compete with a non-profit alternative either...
OK, Seth... show me! (9th comment)
How much money did Craig spend on PR *before* craigslist turned for profit? How much money did he spend up to that point, period? Apparently, not much at all... -- he got the people to pitch in and help and the media to sing him praises.
Did he plan it all out? may well be... but he didn't have to spend any money on PR to get the good press (and free publicity) and see the site grow enormously as a result.
Why couldn't someone else do this and NEVER turn for profit? Use all profits to further the mission like it was supposed to be: start out *and remain* a community service with a philanthropic mindset. The for profit models (such as the current craigslist) couldn't possibly compete.