There's the classic economic textbook example of two hot-dog vendors on a beach that need to choose their location - assuming an even distribution of customers, and that customers always choose the closest vendor; the equilibrium location is them standing right next to each other in the middle; while the "optimal" (from customer view, minimizing distance) locations would be at 25% and 75% marks.
This matches the median voter principle - the optimal behavior of candidates is to be as close as possible to the median but on the "right side" to capture "their half" of the voters; even if most voters in a specific party would prefer their candidate to cater for, say, the median Republican/Democrat instead, it's against the candidates interests to do so.