It's not just that million-dollar-plus campaign contributions likely dampen a fair amount of prosecutorial zeal (reference article posted by Shorebreak in #2 post). Accodrding to an article from The Fiscal Times ("Jail Them Or Nail Them - Was Justice Served On The Banks?"), it's also the fact that Holder and Breuer were
partners for years at a Washington law firm that represented a sort of ‘Who's Who’ of big banks and other companies at the center of alleged foreclosure fraud, according to a report last year by Reuters. The firm, Covington & Burling, is one of Washington's biggest white shoe law firms, and law professors and other federal ethics experts said that federal conflict-of-interest rules required Holder and Breuer to recuse themselves from any Justice Department decisions relating to law firm clients they personally had done work for. Read more
I agree that many people/organizations missed the opportunity to intervene and fix the problem early on before it got out of control. Unfortunately if we brought to justice every one who was involved in the crisis, and those who should have detected it and didn't, it would look like a neutron bomb had exploded: buildings in Washington and across the country would be left standing but they would be empty. That doesn't absolve the people with the responsibility of prosecution from doing their jobs, and furthermore it helps tremendously if they did not previously profit financially from those who are now being investigated.
As far as the apparent disproportion of sentences (40-50 years for insider trading vs occasionally more lenient sentences for murderers and rapists), I would agree that unfortunately the law is not 100% perfect all the time. The magnitude of the pain experienced by the victims/families/loved ones of crimes such as murder and rape is simply unfathomable. No one argues that crimes such as murder and rape are on the same level as financial misdeeds. But the individual impact and lasting effects of the financial destruction caused by the mortgage debacle should not be underestimated either. We the People deserve justice and an explanation of failed prosecution as would be offered in other instances.
Note: Edited @ 1103 to identify the source and title of the article linked