Final answer:
The main factor that has mainly prevented the expansion of health insurance to all Americans is cost. Additionally, politics and diverse beliefs and values about the role of government in providing healthcare have hindered the expansion of health insurance.
Step-by-step explanation:
The main factor that has mainly prevented the expansion of health insurance to all Americans is cost. The high cost of health insurance is a significant barrier for many individuals and families who cannot afford to pay for coverage. Additionally, the politics surrounding health care policy in the United States have also hindered the expansion of health insurance. There are diverse beliefs and values about the role of government in providing healthcare, which has resulted in challenges in implementing comprehensive insurance coverage for all Americans.