Laura McLay asks why is it so easy to forecast the Presidential election and so hard to forecast the NCAA basketball tournament? Nate Silver famously predicted the winner of all 50 states; but if you look at the NCAA basketball tournament, it’s difficult to get much above the low-70-percent range in predictive accuracy. (Silver himself has pointed this out.)
One thing that’s not mentioned, though, is that a basketball game is simply a smaller sample than voting. The basic unit of basketball analysis is the possession; a typical Division I college basketball game might include 150 or so possesions. (Averages per team are at team rankings.) If you let two basketball teams go at it for hundreds of thousands or even millions of possessions, the chance that the better team would win the game would be much higher.
In short, basketball games are subject to sampling error; voting is not.
God plays dice 
I’m not sure it’s only that. I think there are complicated interaction effects between styles of play, so that “what proportion of the time would team X beat team Y” is not clearly a function of statistics of team X and statistics of team Y.