I had to craft the headline carefully. I didn't want to imply that all politicians never fulfill their promises. I didn't want to imply that some politicians always fulfill all of their promises. Well, maybe some do because they make very few promises or they make such generalized promises that it is hard to say that the promise was not fulfilled.
Back to the topic. For the answer see "Secrets of a Pollster" by Thomas Friedman, New York Times, 2009-03-24. The relevant quote is
"[Politicians] never come out of the box and deliver the scale of progress and change they promise — not because they are cynical, but because events conspire against them and they encounter competing power centers. What distinguishes the best leaders, he says, is that they learn from their crashes, adjust, persist and succeed."
