I've thought about that question, and my hypothesis is this:
These mega-insurance companies know that the House or Senate bill would, in the long or even medium term, kill off all private insurers, because of the mandate to cover pre-existing conditions, the multiple mandates to cover mental illness and other health-related conditions, the requirement that they absorb losses but kick back profits to the government, etc.
However...these insurers figure that all of this mega-regulation will kill of their smaller, less well-capitalized competitors first. And then, when the dust is settled and only a about 3 health insurance companies survive, they hope to repeal a good but of the law, either at the Congressional level, or at the level of the enabling regulations put out but the HHS.
They're trying to ride the beast of Statist regulation, rather than taking the more principled approach of killing the beast from the start.
That’s exactly what I was thinking... willfully assist in the destruction of your own industry in the hopes that you will be destroyed last. Brilliant strategy...
Keep in mind that CEOs of a corporation seldom think beyond their expected retirement date. A solution which gets them lots of money from bonuses and stock options short term, but dooms the company long-term, is OK with most.