It won because it was used by Linux kernel first and second because gave it an interface to ease things. https://stackoverflow.blog/2023/01/09/beyond-git-the-other-v...
In fact, it worked so well, I stopped using "svn merge" (which took 5+ minutes on our repository for every merge), and started using "git merge" with git-svn instead (which reduced the merge time to <3s, and even the extra git<->svn sync overhead cost only 30s or so). As a bonus, git also reported fewer "merge conflicts" (svn at the time had issues repeatedly merging from a branch to trunk).
So when I ended up picking a DVCS for another project, git was the natural choice since I already knew it. I imagine there are a lot of developers who started out on SVN and took a similar route to learning git, so having a high-quality Subversion bridge turned out to be one of the critical features on the road to adoption. This advantage in adoption then snowballed via forges like GitHub.
In a parallel universe, had there been a HgHub (with same strong initial iteration as GitHub), I have no doubt that mercurial would have won.
https://www.youtube.com/watch?v=4XpnKHJAok8
He talks about what SVN got wrong, specifically, that it made branching easy and it's the merging that's the important piece. Git won because it made merging easy.