I think the problem is that both parties have moved, the democrats shifting left and the republicans shifting right (and to put the cards on the table, I think the republicans have shifted a lot more rightward than democrats have leftwards). This leaves people pretty unwilling to go for a split the difference approach. Like if you ask people in favour of a centrist position, they're probably picturing someone in the middle of Bill Clinton and George W Bush, and thinking " I could manage with that". But for a lot of the core democratic vote, they see a Romney as the result of drawing a line down the middle and they already decided he was too right wing for them. Or similar, the republicans are worried that splitting that line down the middle ends up at Hilary, who they already didn't like. So while a lot of people express support for compromise and consquently centrism, you'll get into a lot of infighting when you try to decide what the centrist position actually is..