This was a failure of regulation, not just in the US but elsewhere too; banks and mortgage brokers weren't doing their due diligence and were giving out loans and mortgages that people couldn't afford based on their income and other outstanding debts, eventually leading to the 2007/8 financial crisis.

Which should have been a lesson, but five years later, housing prices recovered and ballooned. I don't know why besides increased demand and reduced availability, clearly people can still get mortgages despite the lessons learned from the crisis.