Several reasons:
1) There is still the stigma of geeks being 'uncool.'
2) An adult is usually afraid to poke around and wreck something, kids have no such fear.
3) Reality TV has lowered everyone's intelligence.
4) People are too lazy/cheap to buy a book about learning computers.
5) The OS is not intuitive. I don't find Windows NT/XP etc. or Linux at all intuitive when it comes to customizing, whereas AmigaDOS and MS DOS was pretty logical... so easy to remember commands.
