So I think for a long time there was this big pool of people that could potentially be contributing, but they were really put off by R-help (note: R-help is a notoriously hostile mailing list and was the only way to get help with R in the early days). And then the timing was lucky enough that there were two significant changes that allowed the community to reinvent itself to some degree.
The first of those was StackOverflow. It seems hard to imagine now, but at the time, StackOverflow was so incredibly welcoming and friendly. And I think part of that was that in contrast to R-help, anything would seem welcoming and friendly,
This made me laugh, because I had spent countless hours trawling through installments of the R-help mailing list, and the principal contributors of answers (Brian Ripley, Uwe Ligges, Duncan Murdoch, Peter Dalgaard, others I could once have named without thinking) were often very severe. But after one had trawled through HUNDREDS of installments one couldn't help but be struck by the generosity of contributors who kept answering question after question for months, years on end. To this day I feel an affection for Ripley, Ligges and all (the mere name Uwe Ligges has only to come to mind to make me smile), an affection yet to be inspired by professional contacts who are EXTREMELY friendly and dodge questions like so many bullets.
The whole thing here (this is, of course, the link I want in a convenient place).