I wonder if this is a case of OS Darwinism. The fact that it seems to be hanging on by a thread shows most companies are using and contributing to Linux. I have never used OpenBSD so not sure what it overs over Linux as a Unix implementation?
My understanding is it has a reputation for being very secure, having thoroughly audited source code, and runs on lots of old and legacy hardware. It's also renown for having super thorough documentation. (see Zed's comment)
Judging by comments on this post, I think it is more a case of the maintainers and/or the foundation's management instead of loyalty or adoption of the OS. I believe HN also runs on OpenBSD too, if I'm not mistaken.
On FreeBSD.
That and the foundation's most important projects are probably OpenSSH (absolutely essential, main focus here) and OpenNTPD, rather than the OS itself.