The answer is quite simple, AIG underwrote many large companies, if they were allowed to fail it would have quite possibly caused a cascade failure of many businesses. Without underwriting the companies directors would have no other option but to fold the companies with major problems occurred, and there were many problems at that time.

>Will regulators ever coherently explain why AIG could not be allowed to go bankrupt in September of 2008?
>http://online.wsj.com/article/SB10001424052748704094304575029400270865426.html?mod=WSJ_Opinion_AboveLEFTTop
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>Regulators did not see or ignored the problem as it grew, made bad/wrong decisions early, waited until the supposed critical hour to demand a massive cash influx from taxpayers, directly or indirectly bailed out the very firms they used to work for all while demanding to anyone who would listen that those "too big to fail" were a "systemic threat" to the system itself and required rescuing. Yet here we are, 14 months later, their former firms flush with cash, lending stagnant and they cannot get their story straight on what was the "systemic threat" of AIG. Fishy is not a strong enough description.