I'm going to keep this non-political.
GM and Chrysler have labor costs and financing costs that make them unsustainable. This is due to decades of bad decisions of both the management AND the unions. GM's unions and the bondholders have given in very little in the latest round of negotiations. They likely simply assumed that a democratic president would continue to bail them out. Now they have to go back to the negotiating table and come up with some real cuts or the bondholders are going to be left with basically nothing and the employees will lose a LARGE number of jobs (much larger than if they don't go bankrupt). The bondholders are currently sitting on bonds worth about 22 cents on the dollar. They will likely need to come down close to that value. The unions will likely need to start giving up retiree healthcare, members will have to start paying a portion of healthcare expenses (co-pays, deductibles, percentage of the actual cost), work longer before they retire with a full pension, relax some of the work rules (only certain people can do certain tasks, etc), and many other frivolous things in their contracts. Chrysler is toast.
There is A LOT of work to do.
Oh and the GM situation is VASTLY different than AIG. AIG insured many of the banks losses. Without those insurance payments those banks would have folded or had to be bailed out themselves. Once the banks start falling that would cause other banks to fall. This would happen because banks lend vast sums of money to each other and the losses that would be incurred in that situation would be catastrophic for many banks. Then as those banks failed the FDIC would've had to come up with money to cover the deposits so no matter how you slice it the government was going to be on the hook for a LARGE sum of money. And it's easier to try to stop the problem before it gets out of hand so they bailed out AIG. GM will survive a bankruptcy, the banking system wouldn't have.