As I understand it, you need collective bargaining against insurers to keep individual premiums down. Employers do that today, and if employers just pay out their insurance spend to employees, the employees will get less insurance as a result. A single payer system means the government negotiates on behalf of all of the citizens, allowing it to keep costs down much more than our current system.
Or you mandate that the insurers can only rate on certain variables (e.g. zip code) and/or you mandate a pooling system so that insurers who have many low risk people on their books subsidise insurers who have many high risk people.
