"private markets for health insurance, left to their own devices, work very badly: insurers deny as many claims as possible, and they also try to avoid covering people who are likely to need care." - Krugman
Regulation seems like a pain in the ass, a bureaucratic nightmare and financial sinkhole. The insurance companies will spend tons of money trying to wriggle around regulations and will ultimately wriggle successfully around some of them. The government will expend tons of energy watching what they do and coming up with regulations. A public option, on the other hand, could set the terms, and the insurance companies would have to follow them, compete or fail.
p.s. This is another observation I'm probably not qualified to make. It's just a thought.