Perforce is more the IBM of VCS. Older than it has any right to be. Has quiet "dark matter" support contracts with a lot of companies you wouldn't think need Perforce, but they've been using it for long enough they aren't going to change. Some of those support contracts included complicated forks and homegrown solutions that are so sunk cost as to be nearly black holes (and sometimes so different from baseline Perforce as to be evolutionarily different species).

Well another factor could be that Perforce is a lot easier to use than Git - Actually, would like to think am good with git, but sometimes just wonder how it became so big considering the simple or important things (like check-ins and merges) are so complicated.

It's not even baked into Google anymore.