My guess would be the severe unpopularity of both of the candidates. It’s just not cool to believe in Hillary the same way people believed in Obama. It makes me sad to think that people don’t trust Hillary Clinton, but that’s how it goes when you’re an introvert in politics.
Obama had a real cult of personality going for him, so many people wanted to be a part of his brand. This made Romney supporters ramp up their sign and sticker game. Trump used to have a cult of personality thing going for him, but that all evaporated after the first debate.