According to my prospective there are many complex answers, but put simply, greed is the main reason. Banks wanted big cash flows so their profit ( a percentage of that flow) increased. They had policies for not lending to unqualified buyers, but at the same time they pushed their people very hard to make more loans and they turned a blind eye to the future when the loans would go bad because the people couldn't pay the mortgage on their houses. The previous administration could have enforced regulations on banks, but choose not to. When banks start failing everyone gets nervous and stops investing and the result is a financial crisis (read "depression").
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