Because the only significant predictor for future behavior is past behavior. A man who did not keep his promises to his business associates, or to the American banks whose loans he defaulted on, or to the American public when he made this promise in 2014:"If I decide to run for office, I’ll produce my tax returns, absolutely and I would love to do that," or his wives when he was screwing around on them --these are all easily verified factual events---can't rationally be expected to behave ethically toward anyone else.