The previous popular free code hoster was Sourceforge, which eventually entered its what's now called "enshittifcation phase". Github was simply in the right place at the right time to replace Sourceforge and the rest is history.
1. Free hosting with decent UX
2. Social features
3. Lifecycle automation features
And probably, if the latter, fork that off into a different platform with a new name. (Microsoft loves naming things! Call it 'Codespaces 365 Live!')
And for those who don’t remember SourceForge, it had two major problems in DevEx: first you couldn’t just get your open source project published. It had to be approved. And once it did, you had an ugly URL. GitHub had pretty URLs.
I remember putting up my very first open source project back before GitHub and going through this huge checklist of what a good open source project must have. Then seeing that people just tossed code onto GitHub as is: no man pages, no or little documentation, build instructions that resulted in errors, no curated changelog, and realizing that things are changing.
But VCS has always been a standard-preferring space, because its primary point is collaboration, so using something different creates a lot of pain.
And the good ship SS Linux Kernel was a lot of mass for any non-git solution to compete with.
The big reason I recall was that GitHub provided free public repos and limited private, while BitBucket was the opposite.
So if you primarily worked with open-source, GitHub was the better choice in that regard.