Because you are talking about two different causes. First, to correct you, there was a RECESSION in 1921, not a depression.
Now, what caused the two events?
1) 1921: was caused by a sharp drop off in demand for materials at the end of WWI in conjunction with an increase in supply. The result was the market adjusting for the new demand/supply. Once that settled, the market exited the recession. There was nothing that could have been done to alter this. It was a dramatic, but necessary shift of the curve.
2) Great depression: this was a result of something similar to what we are seeing today. Credit was made to readily available in the 1920's and this resulted in people building up more debt than they could afford. When people started defaulting on debt, the banks failed in large numbers (something over 5000 banks). This is prior to the FDIC, which meant all those deposits the banks had for people... were lost. The combination of the stock market crash and the bank failures caused people to pull in spending, which led to lower levels of goods/services produced, which resulted in more people being laid off.... as you know the cycle continued down.
Bottom line, the two events are nowhere near similar.