Mark Jickling of the Congressional Research Service produced a handy chart of causal factors for the financial crisis. I think there are at least twenty, and I personally would probably endorse at least two-thirds of them as playing some role. The chart does not include monetary policy, which many people argue was too loose from 2003-2005 (I personally am willing to cut Greenspan more slack, because I think that measures of employment were indicating a slack economy). It also does not include capital regulation, which I think amplified other factors, namely the role of the rating agencies and securitization.
My point is not to quibble with the chart. My point is that there were many causal factors involved. We would need to run a lot of experiments in alternate universes in order to sort out the crucial from the incidental.