This is a really interesting overview and perspective by one of the co-founders of GitHub:

So, to sum up, we won because we started at the right time and we had taste. We were there when a new paradigm was being born and we approached the problem of helping people embrace that new paradigm with a developer experience centric approach that nobody else had the capacity for or interest in.

The whole post is worth reading for the history and all ways things just went right for GitHub.