Yes, there were several factors that spiraled up together: the entry of 2 new rating agencies, the new credit enhancement structures, the new types of collateral that the agencies deemed to have "predictable enough" P+I cashflows, and the pension funds and insurance companies' appetites for AAA.

One other factor (from the early 90's, I think), was when getting asset sale treatment for GAAP and RAP (bank regulatory accounting) while still getting debt for tax treatment become standardized. That was a breakthrough.