> Most common software that typical buyers use is available on Mac or Windows
That's not how most people think. Most non-techies are either fluent with "how to use a Mac" or "how to use Windows" and they will just stick with that inertia.
For a lot of people, learning a new OS is an ordeal.
Also possible that people have paid for licenses / apps and thus want to stay with the OS those will run on, instead of having to pay again (if it's even an option).