Paul Krugman has a nice, short post about the persistence of ultra-low interest rates, and how they're taking people by surprise.
His basic argument: It's kind of different this time. The world is transitioning to a permanently slower pace, in part fueled by demographics.
He posts this chart of the growth of the working age population in Europe.
FRED
As you can see, Europe is quickly turning Japanese, with a shrinking population.
This helps explain why European bond yields keep grinding lower and lower, despite all the debt and concerns about the lasting durability of the Eurozone.
The US population is still growing (though not like the old days), but ultimately similar factors are at play.