Why doesn't everyone use OpenBSD for certain things? The first job in tech industry I had was with an OpenBSD shop. The admin was quite security conscious to put it mildly. I'd hazard to say that his attitude and personality fit the OS, which is good. I respected the guy. I think you may be right that OpenBSD is the most secure. Is there a trade off? That is, maybe OpenBSD is a huge pain for some reason? I used this back in like the late '90s so don't really remember many details.
Is there a Linux distro that comes close to OpenBSD? What is it specifically about OpenBSD that makes it more secure and could something similar be done with a Linux distro? These only appear to be brain dead questions, they really aren't. These aren't the droids you're looking for.