Because there's no clear way to transition the US to a single-payer system in a way that wouldn't cause mass rebellion once people see what it entails:
http://www.newsweek.com/quora-question-why-single-payer-healthcare-wont-work-611168
There WOULD be restrictions in terms of people's options and (in some cases) quality of care (ask anyone who is on Medicaid what it looks like). For example, Medicare forces prices lower by simply capping what it will pay for certain services, which turns Medicare into a money loser for many practices who have a choice to A) simply stop accepting patients who use it or B) make up the shortfall by charging other patients more. If you go to a system where everyone is paying Medicare rates, hospitals start closing.
It's not simple - anyone who tells you it's simple is lying. Medical costs in the USA are skyrocketing and it's not because evil billionaires are stealing all of the money; it's due to dozens of different factors that no one is quite sure how to solve. For example: we are already facing a potentially catastrophic doctor shortage. If we go to a government system that caps doctor salaries, do you think that problem gets better, or worse?