Linux

It's the de facto standard for many reasons, none of them being individuals' choices. Microsoft paid and pushed for Windows to become the default OS on pretty much all OEM hardware, they lobbied super hard to push people into using Office, they gave massive discounts on licences for corporations, big and small companies.

It has nothing to do with individual choices, it created the problem you mentioned in your comment though. People just became complacent and ignorant because of that, not the other way around.