Because the government didn't try to "fix" it and allowed market forces to work. We also were not swimming in debt back then.
If you notice, FDR tried to "fix" things and "stimulate" things. Some experts say that actually prolonged the misery.
http://newsroom.ucla.edu/portal/ucla/FDR-s-Policies-Prolonged-Depression-5409.aspx
http://online.wsj.com/article/SB123353276749137485.html
http://www.amazon.com/FDRs-Folly-Roosevelt-Prolonged-Depression/dp/0761501657