A theory on why politics today is so polarized:  Polarization is how the political parties have learned to motivate people.  This is crucial to countering apathy.  And because campaigns can be won simply by increasing turnout, they’ve learned to emphasize the differences.  As a result, a lot more people care, and care more deeply, and are therefore motivated to vote.

I haven’t heard this theory elsewhere, it would be interesting to look at data and test it.