To make this argument, we can point to two factors.
First, health care is an absolute necessity in most cases. Therefore, it is not like most markets in which buyers and sellers have relatively equal power. In many cases, consumers must buy health care and may not even have many choices as to where they will buy it. This necessitates regulation.
Second, sellers typically have a great advantage in information over buyers. Markets are based on the idea that buyers and sellers should have relatively identical levels of information. But health care is so far beyond the understanding of the average consumer that sellers have a great advantage. This, too (one can argue) necessitates regulation.