Because 95% of those people work with Linux, but not in it. OS X is BSD-based but close enough to make developing or supporting Linux machines simple. My entire infrastructure, thousands of servers, are all Linux, but most of the time I'm working on my Windows PC, and only occasionally do I break out my Macbook or Linux laptop. Love Linux, can't stand Apple, and I'm meh about MS except for gaming, but Linux as a desktop/workstation OS is still years behind OS X and Windows in ease of use. That said, I do not nor would I ever run either of those in any production capacity, just Linux.