A mix of factors for me. Firstly, privacy concerns, settings reverting themselves after updates, and the looming threat of Windows 11 were I to get a new PC. Stuttery performance on my already 3 year old laptop at the time (I still use the same laptop. It is now 6 years old and still runs great with Linux). General bloat, driver problems, and instability issues.
I did not make the switch all at once, but thankfully my laptop has two NVMe slots, which made dual booting easier while I got more used to using Linux as my daily driver. Within about a year, I was booting into Windows less and less, and eventually hardly ever once I found ways to use Linux for everything I needed.