Actually there are insurance policies written in some (perhaps many) acquisition situations, to cover liability for past actions which come to light after the acquisition (and sometimes also for as-yet-unquantified liability for actions already discovered). Usually the level of risk is considered acceptable, and the insurer is free to charge pretty exorbitant premiums for this type of policy, even with "deductables" which may be in the 10s or 100s of millions, as it's all being rolled into the financing of the acquisition, and essentially being deducted from the purchase price of the acquisition.
However, with AA's recent history, apparently all the insurance companies which underwrite this sort of policy felt that the risk was great that more large-scale liability situations would come to light down the road -- not to mention that the liability for the Enron and Global Crossing situations isn't even close to being quantifiable yet. If any were willing to seriously discuss underwriting a policy at all, it was in a price range which would have exceeded what any potential acquirer was willing to pay for AA, thus rendering the policy useless for its intended purpose.