My theory is that most people are scared, so they try to belong to a group so that they are less scared. The cohesion in the group is determined by the extent to which its participants are alike. To promote cohesion, people tend to conform to the rest of the group (speaking teh same way, dressing the same way, having similar interests etc.).

Now, if someone does not conform to the group, for example, because they dress differently, or have different interests, then either they are not in the group and should be kept our to promote cohesion (according to these people). If they are already in the group and do not conform, they are either pushed into confirming or cast out of the group, to ensure that cohesion remains intact. This is why some people act like people who are neurodivergent are different, I think.

TL;DR: Many people need to be part of a group to feel safe and feel like people acting differently threatens their group.