One could argue that mortgages given out sub 2.5% became a problem when the fed funds rate went to 2.5% by mid-June. Additionally, a US deregulation wouldn’t explain house bubbles elsewhere at the same time. The truth is that when interest rates are low, debt is more attractive but when/if rates rise, many borrowers and lenders will suffer. This became especially problematic when Fannie Mae and Freddie Mac were able to compete very strongly with government money and gain massive swaths of the market. The other lenders were then pushed into rather unsafe behavior. Thing is, how do you regulate subprime lending? If the answer is just to refuse the “dirty poors” you’ll eventually have an extremely angry and desperate underclass, hence the creation of Fannie and Freddie in the first place. If you outlaw derivative markets you’ll be outlawing an avenue for legitimate hedging, and you’d end up with even greater consolidation of many markets into the hands of even fewer participants. While deregulation did directly lead to the consolidation of the banking system after the crisis, and certainly didn’t help prevent the crisis, I wouldn’t consider it a primary cause.