And don't forget government policy and meddling politicians. If anything, that played one of the largest roles. Legislation meant to make "everyone" a homeowner by removing barriers to home ownership played a large role. Fannie Mae and Freddie Mac policies drove bank lending in large part. Originators were writing crap and Fannie and Freddie were buying it from them at a feverish rate without a second thought.

This interview with Thomas Sowell covers a few specifics. https://m.youtube.com/watch?v=E1KwkScA540