I think the center makes a lot of America warm and fuzzy, but doesn't get them to the polls. With trust in leadership at abysmal lows, a candidate marginally aligned with your views, but solid in holding those views is a better bet than a candidate who is for-sure everything you like, you bet, promise, cross-my-heart and pinky-swear...to everybody.
If a small crowd at a town hall event or a brief twitter storm shake their principles, why would you expect they could stand up to K Street in D.C.?
ETA: I don't think its an American phenomena, however. I think the most successful centrist (local Overton Window) in the last few years within the OECD is Macron. How's that going? It leads to paralysis because the nominal opposition knows it can show a pretense of dialogue while simultaneously going full obstruction (another example: Obama's terms). One man's total jerk is another man's unwavering example. People want resolve, they want someone who's saying "we're doing it this way, by the way there's lots of positions to fill to get it done, so roll up your sleeves and get on it or piss off." How's Theresa May doing trying to find a cozy route through that doesn't make anyone too mad?