At a deep level, our experience of reality has become wholly moderated by mass media. Reds and Blues are watching different channels, and thereby experiencing different realities. It's as simple as that.
Nonconformance to a media narrative is punished by all, in a distributed fashion. If you express an independent point in a Blue flavor, you will be attacked by both the ever-present Reds as well fellow Blues for breaking rank (and vice-versa, obviously).
Even this comment itself would be better if I tied in some basic examples. But they would inherently reflect my own filter bubble, opening my point up to partisan scrutiny looking to reject self-reflection xor continue caricaturizing the others.