It's hard enough to find Linux admins, even in major cities. There are people who will put Linux experience on their resumes, but almost nobody who can boast more than casual home use. We ended up giving up on finding a Linux admin after a few months of the job posting collecting dust and decided to just hire someone fresh out of college a train them from scratch.

Then you run into the problem that BSD admins are even rarer, and AIX/Solaris even rarer than that.

Someone with experience with Linux could likely find their way around any UNIX like system but given the choice we would rather deal with some awkwardness in a familiar environment where admins have far more experience and we can leverage our exiting infrastructure, scripts, and config management.