The answer is in front of our faces. It's because American consumers, whose spending is 70 percent of economic activity, don't have the dough to buy enough to boost the economy - and they can no longer borrow like they could before the crash of 2008.
If you have any doubt, just take a look at the Survey of Consumer Finances, released last week by the Federal Reserve. Median family income was $49,600 in 2007. By 2010 it was $45,800 - a drop of 7.7 percent.