Shorter Dean: Because the Justice Department let them:
[In a real investigation] [t]he people who put together some of the worst mortgage backed securities would be asked if they were really dumber than rocks and had no idea that many of the mortgages being put into the packages were fraudulent. If the prosecutors could demonstrate evidence of intelligent life at Goldman Sachs and Morgan Stanley they would then ask the lower level people whether they wanted to spend years in jail or would rather explain why they thought it was a good idea to put tens of millions of dollars of fraudulent mortgages into mortgage backed securities. This would presumably lead to testimony against higher ups at these investment banks. …
There is no guarantee that these sorts of efforts would have landed top executives of financial firms behind bars. However there is no evidence that the Justice Department even began this sort of investigation. At the least, such an investigation would have resulted in prosecutions of lower level actors who clearly violated the law in issuing and passing on fraudulent mortgages.
As [Neil] Irwin said [link added -- Lex], bad business judgement is not a crime. However, it is a crime to allow bad business judgement to lead to fraud. Clearly fraudulent mortgages were a major factor in propping up the housing bubble. No one went to jail for this crime.