Because the US dollar is still the reserve currency and, therefore, we still benefit from the "flight to safety" that occurs when investors get scared.
Investors simply know of no safer place to hide.
So despite the trillions of dollars that the government has borrowed and printed to bail out troubled financial institutions and help stimulate economic growth, demand for Treasury bonds only grows.
And as demand for Treasuries grows, the US economy benefits in numerous ways: 1) prospective homebuyers get cheaper mortgages, supporting housing prices; 2) existing homeowners can refinance into lower-rate mortgages, freeing up disposable income for other purposes; 3) businesses pay less on their outstanding loans and credit lines, freeing up resources for new capital equipment or new employees; and 4) the federal government pays less to service its massive debt load, lowering budget shortfalls and providing cover to continue its reckless spending.
So while the crisis in Europe has also led to some negative side effects in the US (stocks are down 12%, bond spreads are widening, consumers are more cautious), the resulting decline in interest rates is coming at a very fortuitous time in our economic recovery.
But lower interest rates also have a down side.
Falling interest rates will undoubtedly encourage borrowing and spending over saving and investing. This incentive structure is what caused the financial crisis in the first place.