"And Chrome OS just works, reliably and securely (by mainstream OS standards)."
I think this is it in a nutshell. People know that google's spying on them but they believe Windows does the same AND borks their data & machine. With Chrome your data & machine remain relatively unborked.
Then you factor in the fact that people spend more and more of their time on other people's schedules. They have a limited amount of time write their email/video chat with g'pa/reap the benefits of porn, etc. They don't have time to futz with anything, even if they have the skills and desire to do so. Shit has to work the first time, every time and personal preferences aren't even in the equation.