The git wrapper was never fully supported and had some rough edges (I think it was only ever a 20% project, and also its, like, really old). And the customized mercurial has been around for more than 7 years, I think close to a decade (the client I'm using right now is turning 7, and it wasn't my first one).
I'm going back starting on monday, so I'm curious to try out jj.
In the past 10 years it's all been github and gitlab, and their code review tools are so painful, specifically w.r.t. tracking discussions across revisions. I never felt excited to try out jj because I was afraid it would that situation even worse.
Yet another reason is that the .git directory is considered a documented API and several other tools and libraries depend on it (e.g. JGit and libgit2). So any new features for Google would need to be made to those tools too if we wanted things built on them to work.
We also consider Mercurial to have better UX.