NikenDiana, one possible answer is because people are used to what they're used to. Changes in their environment may cause them to feel worried.
Social psychology proposes one answer: "in-group" vs. "out-groups". The former would be the people you associate yourself the most with. While the latter are the rest of the world.
To overcome this, the person needs to expand their view of the world, understanding and knowledge. Sadly, many opt to impose tolerance and understanding. In another hand, it's also the case of "leading the donkey to the well, but you can't force it to drink".