Because Taxes. Personally, as an American, I'd be fine with paying a little extra in taxes if it meant I didn't have to worry about going into debt over a major surgery.
Wouldn't we all pay for it and who wouldn't use it? Since it would be payed for through Taxes anyone that's working would fund it and anyone that visits a Doctor or takes Prescription Medication would make use of it. Rich, Poor, or Unemployed we would all make use of it.