We've gotten an explanation from Rep. Steve King (R-IA) for why he was the lone vote against acknowledging the role of slaves in building the U.S. Capitol. He did it to protest "a several year effort by liberals in Congress to scrub references to America's Christian heritage from our nation's Capitol":

Our Judeo-Christian heritage is an essential foundation stone of our great nation and should not be held hostage to yet another effort to place guilt on future Americans for the sins of some of their ancestors.

So there you have it.