Here is a wiki on the Great Recession and the post-recession boom.
Like the Great Depression, the Great Recession was characterized by a large increase in money demand. Banks, especially, liquidated assets for cash and have been holding money ever since. That is what quantitative easing was designed to do: bailout the banks by having the Fed buy the securities they were holdings. The Fed lowered the Federal Funds interest rate to near zero during the recession (which officially ended in the summer of 2009). So, the Fed’s expansionary monetary policy, which began with QE3 in 2012 (with the economy still in recession, unofficially) could not lower short-term interest rates. Banks are still holding huge excess reserves instead of lending normally. Only in the last few years have banks begun to normalize their credit expansion. And now there are signs of another crisis.