https://www.techspot.com/news/71732-new-steam-survey-shows-windows-7-has-suddenly.html
TL:DR
Worldwide, Windows 7 remains the most popular version of Microsoft's OS, but that's primarily due to its huge number of enterprise users. The opposite had been true of Steam gamers; so, why the sudden change? It turns out that it's mostly down to one game: PlayerUnknown's Battlegrounds. Or more specifically, it's a result of the Battle Royale title's popularity in China and the number of people who use Windows 7 in the country.
TL:DR