The Red Tribe and Blue Tribe have different narratives, which they use to tie together everything that happens into reasons why their tribe is good and the other tribe is bad:
When an issue gets tied into a political narrative, it stops being about itself and starts being about the wider conflict between tribes until eventually it becomes viewed as a Referendum On Everything. At this point, people who are clued in start suspecting nobody cares about the issue itself — like victims of beheadings, or victims of sexual abuse — and everybody cares about the issue’s potential as a political weapon — like proving Muslims are “uncivilized”, or proving political correctness is dangerous. After that, even people who agree that the issue is a problem and who would otherwise want to take action have to stay quiet, because they know that their help would be used less to solve a problem than to push forward the war effort against them. If they feel especially threatened, they may even take an unexpected side on the issue, switching from what they would usually believe to whichever position seems less like a transparent cover for attempts to attack them and their friends.