Ever-increasing dissatisfaction with the government combined with the illusion of change provided by swapping political parties. The presidency has been passed back and forth between the parties for the last few decades as a result; the candidates really don't matter all that much, because voters aren't voting for the candidate, they're voting against what they see as the current status quo.

I'm expecting an acceleration, actually, as the current generation, with expectations shaped by the internet era, becomes the dominant voting force; which is to say, within the next twenty years, all presidents will become one-term presidents, and third party presidents will become viable contenders after the collapse of the major parties into infighting, unbolstered by eight-year terms in which the losing party reconsolidates its coalitions.