I wanted to select all of the above. Although, I would not have qualified the home buyers as poor.
Yes, you had people buying homes as investments to sell in a couple of years and no real chance of ever paying off the loan. Yes, you had banks giving loans to people that could not afford the homes. Yes, you had regulation and enforcement that forced banks to give loans they didn't want to give (on top of the other bad loans they did want to give).
Like any tragedy, if you remove one of those factors you prevent the event. I would have preferred less government regulation.