Having the kernel as an anchor customer definitely helped, but there's also the reason that Linus wrote git in the first place, and that's that every other competitor was horrendously slow by comparison.
I don't think performance was the main motivator; from what I recall it was mostly that Linus felt that the subversion model was "completely broken" and that there weren't any good distributed "bitkeeper-like" tools out there (and then, suddenly, there were three).
"When we first started working on Mercurial, we found that it was slower than Git in several notable areas. To narrow this performance gap, we’ve contributed over 500 patches to Mercurial over the last year and a half."
https://engineering.fb.com/2014/01/07/core-data/scaling-merc...
Personally I can't recall any serious performance difference after I switched from mercurial (which included some large-ish repos) to git, but it's been quite a few years ago and perhaps I just forgot.