This isn't really the gotcha moment you think it is. Primaries are, well, mostly a farce and the people that show up for the primaries are the ones that often have the most free time or ability to do so. Retirees, wealthier individuals and so forth. There's no better evidence of this fact than candidates like Bernie Sanders having extremely high popularity among the younger generation which tapers off the older people get. There's a massive and growing generational divide that's only going to get better once the voting block that enables them starts dying off.

Assuming that the primaries represent who will actually win a presidential election is as much of a mistake as assuming that polls are accurate when it comes to election day.