Greed, pure and simple. Greed on the part of the borrowers, who thought they could get something for nothing. Greed on the part of the mortgage brokers, who wanted to keep the fees rolling in regardless of the creditworthiness of the customer. Greed on the part of the investment banks, who packaged and sold the mortgages without pricing them in properly to reflect their risk. Long and short none of the people participating in the fiasco are innocent.
It’s not greed, silly. People are greedy when the economy expands, too. The problem was that the government distorted the market, and no one knew when they should stop lest their greed be thwarted.