These are the reasons (from a comment I left over at JOM):
1. Individuals have a right to form private insurance contracts without undue interference by federal and state government. These rights are already being violated and will be even more seriously violated under the proposed enormous increase in government interference.Also, you have to look at the problem you are trying to solve. What is the problem? 1. Inefficiency in the health care industry. 2. The approximately five million Americans who can't afford health insurance and don't qualify for Medicaid or Medicare, some of whom may be good people in dire straits and therefore deserving of a little assistance.
2. Insurance companies have a right to do business without having to compete against a government insurance agency which sells its product cheaper and at a loss and which funds the program by forcing Americans who don't want it to pay for it against their will. People will pay for it against their will every April 15 via the IRS, which will use force to collect, even from Americans who do not participate in the government insurance plan.
3. Medicare, Medicaid and Social Security are broke, so it would be foolish to create yet another big government welfare program and expect it not to go broke.
4. As in Canada and England, U.S. government health insurance will have to ration health care because of the enormous pressure to avoid bankruptcy. After the government has driven private insurance out of business through unfair practices, this means that all Americans will have their health care rationed. It follows that Americans will suffer and die as a result of government infringement of their liberty to form private contracts.
These problems have many alternative solutions which do not reflexively resort to big government programs. Let insurance companies sell across state lines, for example. De-regulate the coverage they are required to provide, so that catastrophic insurance/HSA packages become more prevalent, tort reform, etc., etc., etc. There are loads of great ideas which increase liberty, prosperity, and charity. There are loads of experts in this field who understand the value of liberty and the burden on it and prosperity posed by big government and who have great ideas on how to fix the health insurance problems. (Just one example: Michael F. Cannon.) There is no reason to opt for government insurance, and there are plenty of reasons not to do so.