It was also lack of controls throughout the system- securitization, fractional reserves & you name it. They were playing the "I'll buy your shit if you'll buy mine" game in a round robin fashion of turning mortgage liabilities into bunk bond assets so that they could offer collateral to borrow in the repo market to keep the whole thing afloat. When repo lenders pulled the plug they were screwed because their bonds weren't paying off. They couldn't meet the overhead to keep the doors open unless they were underwriting bonds that investors no longer wanted to buy. All the hedging in the world doesn't matter if your counterparties are in the same boat as you, because they can't pay, either.
If their investment bank went down the tubes they still had the money made driving it into the dirt.
More technically, CDO's & such mathematically turn risk localized to individual loans (ie a borrower defaulting) into systemic risk of the market performance in general. That's where they were getting the free lunch from, at least when the market was flat or going up. Of course when the market goes down everyone is screwed instead of the just some who just happened to have the worse loans. That's why the system as a whole needs regulation against these sort of bets due to the consequences of volatility beyond just the speculators.
It's a difficult and expensive job, and degenerates are just the sort to cut off this necessary service in exchange for ethnic social status.
