For me it was a combination of factors: Windows has been going down the shitter for at least 10 years now, FOSS software has been getting better and better, and I've learned to use more FOSS tools as I grew tired of dealing with Windows.
If I had to point at one project that made me go "Wow, this is amazing", I'd say ffmpeg. Even in my Windows days, I've always enjoyed digital preservation, when I discovered ffmpeg around 2015 it was an eye opener, so many features, so many options, I've been using it on a daily basis ever since.