All of these reasons boil down to "if it ain't broke" and "that's what we're used to".

Switching VCS for a project of this size is always complicated and OpenBSD devs are famously "old school" and conservative with their software choices.

I used to use CVS before switching to SVN and later DCVS like Mercurial and Git. The claim that "it is unlikely that any alternative versioning system would improve the developer's productivity or quality" is absolutely laughable IMO.

This is especially true nowadays where CVS support in modern tooling, if it even exists, is usually legacy and poorly maintained.