Because a business cycle contraction occurred? If GM had been allowed to fail, other companies would have filled the gap in the market, and new businesses would have emerged, most likely generating even greater tax revenue over time. While there may have been some short-term thumb twiddling, the market would have eventually created new jobs elsewhere, jobs that would have been even more productive. There's a great opportunity cost to "saving" millions of unproductive jobs.
> A study by the Center for Automotive Research found that the GM bailout saved 1.2 million jobs and preserved $34.9 billion in tax revenue.
The whole point was to avoid the economic fallout of a GM collapse (and with it, yet another bank for the FDIC to deal with).