Their counterparties are all sophisticated institutional investors and these things are generally traded on a buyer-beware basis. Sure there may have been some actual wrongdoing that went unpunished, but the nature of these transactions is that the cases are likely be civil rather than criminal.

The real reason for the lack of convictions is that there just weren't many cases of big bad wall street bankers defrauding ordinary people. That's typically what results in criminal convictions because 1) the law is far more protective of unsophisticated individuals than of institutional investors and 2) prosecutors have a much easier time winning cases and also benefitting politically from pursuing cases with relatable victims. And the reason why people wanted bankers to go to jail is because they thought wall street bankers did bad things to ordinary people. In reality though, the best cases against Wall Street folks are ones where the victims are also institutional investors (which is often included within the broader definition of Wall Street). The traders at Goldman Sachs outsmarting the traders at AIG just doesn't make for a particularly good fraud case, the same way Enron defrauding individual investors including their employees. And from the public's perspective, they are all part of the same class of people. And where ordinary people were directly involved, they were the ones defrauding lenders (and by extension, Wall Street and so on) by lying on their loan applications.