People wonder why e.g. New York Attorney General Andrew Cuomo is seeking only civil, and not criminal charges against Bank of America executives. The "legal" reason seems to be the required higher standard of proof.
A Ponzi scheme, BY DEFINITION, is illegal. Individual (early) investors can "make out," but they can't AS A GROUP. The sum of gains has to equal the sum of losses. Meaning that if ANY money is diverted, someone will lose.
On the other hand, the government lost its criminal case against the two Bear Stearns hedge fund managers. That's because they "might have" eventually made shareholders whole. Granted, this possibility was rather slim. But "slim" is not the same as "none." There was clearly a zone of "uncertainty" in (jurors') minds. And "uncertainly," means "not guilty beyond a reasonable doubt." That's why a prosecutor would use civil charges which only require proof by "preponderance of evidence," basically a 50.1% "chance."
This is particularly true of the models that use "Ito's calculus" or "partial differential equations" which most jurors don't understand. Which is to say that they will be open to arguments that the MANAGERS didn't understand the models either. And if there is a lack of proof of "scienter," the managers must be found "not guilty" under common law. Even if they're "not so innocent."