Is the cause of the 2008 financial crisis, something about it not being illegal?

There were a lot of illegal acts that contributed to the crisis- securities fraud and violations of banking regulations.

But there was a general disregard for prosecuting anyone. The Department of Justice did not file criminal charges against anyone.

Yes but was there something in particular that was not illegal which contributed to it?

Securitization of bundled mortgages was not illegal, and is still done today. Credit Default Swaps are not illegal, and how they were reported for accounting purposes at the time was not illegal.

Greed unfortunately is not illegal.

David X. Li’s Gaussian copula function being misused was a big part of it.

Repeal of the Glass-Steagal act is likely the move that set the stage and started everything in motion.