I clicked on this link thinking "timing and product quality", so I was satisfied to see that GitHub co-founder Scott Chacon credits it to "GitHub started at the right time" and "GitHub had good taste".
git won because linux used git, and the vast majority of open-source code was written for linux. Simple as that. GitHub won because it made remote collaboration on a code base easier than anything else.
I think that if github hadn't come out something other than git would have one. While git did have Linus behind it, the others were objectively better in some way and working on the areas they were objectively worse, and eventually the advantages would have got everyone to switch. However the others never had anything like github - even 20 years latter they still aren't trying (rumor is they are not dead)
