Because humans are creatures of habit, and as long as stuff works for them, even suboptimally, a lot of people aren’t inclined to change their habits, even if it might mean a change would improve their lives. As long as sth is “good enough”, there simply isn’t an incentive for change.

Also, since there isn’t a huge corporation behind Linux, there is literally no direct advertisement for it, so people who aren’t into tech simply never will get in touch with it.