In The Big Short, Michael Lewis describes how the rating agencies and banks effectively committed mass fraud (by declaring a package of loans which were all BBB level AAA). Eventually, the mortgage bubble burst, and millions of jobs were lost. The banks effectively caused all this by creating loans worth less than dirt, and quickly selling them off. However, not a single banker was prosecuted. Why?
Banks weren't prosecuted for the financial crisis because there wasn't much they could be prosecuted for that wouldn't result in bigger blow back against the government. The banks were giving mortgages to all sort of people that weren't actually qualified, but the government was pressuring them to issue more mortgages because increased home ownership looks good for politicians.
The problem really was the rating agencies who certified packages of mortgages as AAA without really doing any good investigation. At the time though there wasn't much regulation on the debt trading market, and everyone was caught up in making so much money that investors, rating agencies and the government were all happy to essentially look the other way.
When the bubble popped and everything was beginning to come crashing down the primary focus was on preventing the entire market from crashing along with the mortgage debt market. Any prosecution would be at the corporate level resulting in fines of millions to companies that were losing billions, and wold likely exacerbate the crash rather than mitigate it. Now that time as passed and everything has pretty much settled it could be possible to prosecute come companies, but this would be an extremely high profile case and far from a slam dunk for the prosecution. Therefore, its unlikely and DA would risk their career pursuing this.