The plan attracts those who should be paying a higher rate but charges a much lower number.  At some point that does not work any more.  As the plan fills up with high risk high cash outflow people, they need to start charging more which means the lower risk people leave and you are left with just high risk people.  The below link is a nonpartisan group saying the increase in costs has already gone up 25% in just one year.   
http://www.ncsl.org/research/health/health-insurance-premiums.aspx