Agreed, I think the sheer size of the crisis relates back to deliberate policies to support sub-prime lending. The idea was to help people to afford homes even though they would not normally be considered a good risk.
But opening this up created a vast new market, like adding a wider bottom tier to an already giant pyramid. Along with the vast size came even higher risk, since all these people would be more liable to default if the economy tanked.
Even still because we are accepting more sub-prime liability, it goes beyond poor people to others with better incomes but are buying into a more expensive neighborhood. Now people of all incomes are getting sub-prime loans, and this feed into rising housing prices.