Momentum and lack of time and awareness.

Most people don’t associate Windows as anything they can change. It’s what came with their PC, it currently works, it does what they want, the only thing they do is play that game and maybe surf the web… and even if they do know and care, when they get home late from work, the first thing they think is “let’s eat and then unwind with a game and go to bed”.

If more computers came with Linux from the factory, you’d see a better uptick because it’s done, it works, etc.