The great deppresion was caused primarily because?

Nine thousand banks failed during the months following the stock market crash of 1929. It is far too simplistic to view the stock market crash as the single cause of the Great Depression. A healthy economy can recover from such a contraction.

What made the Great Depression "Great" was the government response. Constant changes the regulatory environment, tax increases, massive deficits, and failure to let the market correct paralyzed the economy in its depressed state for 15 years.

Both were caused primarily by an over expansion of credit rooted in loose money supply. The monetary response to the current recession has been different. Rather than tightening to force the market to bottom, the Fed has maintained low rates in an effort to re-inflate the bubble conditions. Hoover/Bush & FDR/Obama responses are similar as all tried to spend their way out of the problem.

1929 crash:

After WWI, Britain reset the pound to the pre-WWI level even though their money supply had far exceeded pre-WWI levels. In an effort to slow the flight of gold from Britain, the US federal reserve (led by Benjamin Strong) lowered interest rates. As always, artificially low interest rates caused massive distortions in asset values. Money flowed into the stock market and people who would not normally have been stockholders bought stocks in place of other investments that would have yielded better interest rates absent fed policy. Margin was used excessively because the real cost of leveraging was distorted by fed interest rate policy.

The fed continually lowered interest rates all the way into 1929. When the bubble popped, they tightened policy and raised rates. This contributed the deflationary spiral; however, the deflationary spiral could not have been as severe without the loose policy during the bubble.

2008 crash:

Beginning in the early 1990s, the federal reserve (led by Alan Greenspan) lowered rates while monitoring consumer prices as indicators of inflation. They ignored bubbles in the stock market directly caused by their inflationary monetary policy. When the stock bubble popped, they lowered rates further and pushed misdirected investment towards other assets - most commonly housing.

After the attacks of 9/11/2001, the fed pushed rates to 0 (long term rates were effectively negative and continue to be).