Because the Fed failed to adequately police its member banks.
That's true, and I would agree with you on that.
However, though deregulation certainly contributed to the crisis, the fundamental problem was the mis-pricing of credit. The Fed sets the price for credit. The list of the Fed's mispricing of credit and it's proclivity to bailing out the financial markets is long, and fundamentally was at the heart of this problem. Of course, the Fed denies this.
Even Wall Street - which deserves a whole lot of blame - took much of its cues from the Fed and the market in general. Institutions such as pension funds and insurance companies have return targets they must hit. Because the Fed kept interest rates so low for so long, institutions began reaching for yield, and
demanded the structured products from Wall Street. For a time, European institutions demanded nothing but structured products from the Street. So Wall Street was happy to comply. Institutions were yield pigs, willing to pay an extra 20-30 basis points for AAA-paper above Treasuries.