Well given 9front managed to do a git rewrite and switch to it (they used mercurial before) there's at least precedent in doing so.

With 9front probably not having the option of using upstream git, while OpenBSD seems mostly concerned about infra maintainability (and security).

It kinda is licensing as OpenBSD is trying to remove GPL software. (quoted from openbsd.org/policy.html)

The GNU Public License and licenses modeled on it impose the restriction that source code must be distributed or made available for all works that are derivatives of the GNU copyrighted code.

While this may superficially look like a noble strategy, it is a condition that is typically unacceptable for commercial use of software. So in practice, it usually ends up hindering free sharing and reuse of code and ideas rather than encouraging it. As a consequence, no additional software bound by the GPL terms will be considered for inclusion into the OpenBSD base system.

For historical reasons, the OpenBSD base system still includes the following GPL-licensed components: the GNU compiler collection (GCC) with supporting binutils and libraries, GNU CVS, GNU texinfo, the mkhybrid file system creation tool, and the readline library. Replacement by equivalent, more freely licensed tools is a long-term desideratum.