This is a summary of this article.
The Grouping Problem:
1) Denial for preexisting conditions is a requirement for anyone buying health insurance individually rather than through a group such as an employer. (Think of it like trying to buy homeowner’s insurance after your house has already burned down.)
2) The decision of who buys insurance on the individual market rather than through an employer is not based on merit or ability to pay, but is essentially arbitrary. Therefore, any health insurance free market excludes some people arbitrarily.
3) You can’t ban denial for preexisting conditions without also forcing everyone to buy health insurance.
4) Forcing everyone to buy health insurance is universal coverage. So there are only two choices: universal coverage or denial for preexisting conditions for individuals.
The explanation of exactly why each of these things is true is of course provided in the main article, and I hope you will read that before making a judgment about the validity of these claims.