I'd say convenience is the topmost reason. For most, it's not a matter of choice, Windows is preinstalled, they have been provided experience with it in schools, and Microsoft is often the standard choice in offices, with doc/docx, xls/xlsx being the de-facto standard document formats. For many, Windows is synonymous with a PC. Windows, in this context, is a low-level utility, beyond the boundary of care, like the manufacturer of car tyres.