Because the requirement to cover preexisting conditions allows young people to carry cheap or no coverage and only buy a good plan if they need it. Which means healthy young people aren't propping up the system by paying for all the older people and people with chronic conditions.
This was Obama’s plan all along. Create a system doomed to fail so America loses faith in the entire concept of health insurance. The big gamble was that the Dems might not control Congress when everything turns to shit.