Because I wanted to use a computer and OS is needed. When I got a first computer, I didn’t even understand what OS is. Then researching further, I thought Windows is just another distribution. I tried it, and it was confusing. Linux Mint I immediately liked, so that’s what I stuck with.
No special reasons, I am just used to Linux.
I guess this would be the case for more people if Windows wasn’t the default as it usually is.