Gee, I don't know. Maybe it has something to do with the government giving away 700 billion dollars to financial companies burned by rampant bullshit fantasy valuations...
The banks have paid back all that money and the US treasury made a profit. The auto industry has also paid back their portion, but the government still holds some stock in the companies, so people can debate whether that portion of it is really "paid back".
http://en.wikipedia.org/wiki/Troubled_Asset_Relief_Program
and http://www.financialstability.gov/latest/tg_10082010.html
OK, then. When the web 1.0 bubble burst in 1999, a whole lot of people lost their money. Bullshit valuations make the stock exchange a snake-oil market.