It was fast, stable enough to work for months or years without crashing, secure, didn't need frequent re-installs, didn't need constant cleaning / defragmenting, didn't have (too many) anti-features nobody wanted or used, it just did what you wanted it to do.

It definitely helped that it existed in an era of app monetization through targeted advertising, as opposed to monetization through bloatware, start page hijacking and completely unnecessary toolbars.

8 was when things started going sideways. 10 was not bad, but it already started the "Microsoft knows better" trend, with automatic updates you couldn't turn off and files you couldn't touch, even as administrator. 11 is what it is.