Alex Pollack reckons finance professionals did believe housing prices could fall, but just not on a national level. The data used to stress test mortgage assets was based on the experience of Texas and other oil-patch states in the 1970s and 80s. It provided an instance of a housing bubble that led to falling house prices. The problem was, since the Depression, house prices had never fallen on a national level. There existed no data that contained a large and positive correlation of home price across different regions and also had prices falling. This is the limitation of istorical data; you use the past to predict the future. When you enter a new regime you are left with your own ad-hoc judgement. Rather than take on that sort of responsibility, most prefer to base their assumptions on historical data.
Read More: The Economist