Arnold Kling examines an FDIC document on the risk weights of different bank assets, and finds that securitization dominated the mortgage market because of regulatory arbitrage, pure and simple:
The higher the weight, the more capital the bank has to hold against that asset. As I read table 1 and table 3, if you originate a loan with a down payment of 20 to 40 percent, the risk weight is 35. But if you buy a AA-rated security, the risk weight is only 20. So if a junk mortgage originator can pool loans with down payments of less than 5 percent, carve them into tranches, and get a rating agency to rate some of the tranches as AA or higher, it can make those more attractive to a bank than originating a relatively safe loan.