They were able to predict Trump winning. All of their data showed it. The polls showed Trump winning everywhere. What they failed to do was trust the statistics. The polls show Trump as favorable, and they say "yeah but he won't make it past the first state." The polls show Trump beating other candidates and they say "he has a ceiling of 30%". Other candidates start to drop out and polls still show voters rallying behind Trump, and they say "the polls are wrong, the voters will go to Cruz".

I'm not a Trump supporter by any means, but 538 went way out of their way to ignore every poll that was put in front of them in favor of punditry.