There are a few reasons. One reason is that people feel threatened by what they perceive as a threat to their own beliefs and values. For example, if someone believes in something that is traditionally seen as harmful or undesirable, they may feel threatened by that belief. They may also feel threatened by the idea of others accepting and supporting that belief.
Another reason is that humans tend to default to negative views of other people and cultures when there is uncertainty or conflict. For example, if we don't know much about a different culture or community, we may automatically assume that those in that culture or community are bad. We may also assume that their beliefs are harmful and will lead to conflict and disaster.
Finally, humans can be threatened by the idea of others changing or betraying our trust. When we rely on others for support or information, it can be unsettling to think that those same people could suddenly turn on us or betray our trust. This can create fear and insecurity in us