There's a ton of competing theories that like to point out that previous depressions lasted a year or two while the Great Depression went on and on because of all the government interventions that prevented the economy from being able to recover.
Luck and variance are pretty good explanations, though people tend to hate them. The idea that every thing in the universe has to have a specific cause and effect plagues a lot of "analysis" in today's world. We simply aren't built for accepting luck.
