Even as late as 2004, notice the broad crossover between the median Democrat and median Republican. And then notice how wide the divide is today.
Not only are the median positions of both parties further apart, but both parties have also shifted farther to their respective extremes in the last 13 years. The middle ground is much smaller, and to my eye it looks like the Democratic group is somewhat bigger than the Republican. You can see the same thing in the breakdown of the vote by states and counties; but since political commentary is not my genre, I'm going to avoid going any further down that rabbit hole.
But I will say that the internet, social media, and the media we consume on TV have allowed us to live in echo chambers where we are not really hearing much from the other side. We talk to people who think like we do and who tend to confirm that we are correct in our beliefs. That constant cycle of reinforcement makes our positions even more hardline, to the point where we trivialize or disparage the other side. It has seemingly become acceptable for an American congressman to say that he doesn't feel sorry for those killed in the mass Las Vegas shooting because they were likely Trump supporters and against gun control. And for white hate groups to blatantly and publicly espouse racist positions. Antifa groups can call for the random killing of white people, simply for being white. And fewer than 30% of Millennials think that democracy is clearly the superior system of government.