The answer is that the US Bankruptcy laws are the fairest laws in the world and transparency in the bankruptcy process is miles ahead of any nation. So the reason not to use the process that every one else uses is to provide special protection to a creditor class not afforded under the current law..... in this case the UNIONS!!!
However, GM management gets the last laugh. If this was a traditional bankruptcy, you'd bet every effort would be made to find every bit of liability so that the debt can be settled in bankruptcy at a discount rather than later. GM management intentionally withheld these problems from the government so they could emerge from Obamabankruptcy stronger (although this was a facade). Thus they could enjoy continued employment and bonuses. Then they retire when the sh*t hits the fan.
This is cronyism at its best which is usually seen in third world banana republics.
/SARC for them wot needs it