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.
git, mercurial, and bazaar were all released at pretty much the same time (within weeks of each other in March/April 2005). AFAIK all in response to the whole Bitkeeper situation at the time.
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).
Linus was looking at monotone as an alternative to BitKeeper, but in his words "performance was so horrendously bad that [he] tried it for a day and realized [he] cannot use it": https://youtu.be/4XpnKHJAok8?t=711
It was very noticeably slower on all but the smallest projects. I started with it but switched to Git later for some open source work and that was one of the first impressions I had.
According to Facebook Mercurial was faster for them, but that was in 2014 and they had to put work in to actually make it fast. Their use case is rather outside the mainstream of course.
"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."
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.
