I have set up a Linux computer for my dad just this week. As the other are saying the biggest hurdle is support. Mostly the part where reliable automated update mechanisms don’t exist. The system I created has a WM & a panel and it’s even simpler than chrome OS, but here is no way my dad could deal with a message, where apt would ask him, if he wanted to update to the maintainer’s grub version. Making an update script is very easy, but whether you use Debian stable, Ubuntu or Arch eventually some kind of intervention is going to be required beyond just typing sudo passwords…

Though truth to be told Windows has a lot of garbage problems (perhaps even more than L) as well, it’s just people have gotten used to them after 20 years of NT on desktop.