Usually... a religious background. While most major religions do talk about inclusivity and helping others... They're also filled with stories and sometimes even instructions of how to destroy those that don't believe the same way.

There are religious leaders that push agendas of hatred because they have the power to do so and they're harbor that in their own heart, surrounding themselves with religion as a shield.

Because they're fed lies and projection that those people are coming to ruin their way of life. And it's very hard to undo all of that reinforcement, a lot of dissonance and regret and introspection to navigate if they do along with all the inevitable lifestyle change required to leave that environment. That's why even the ones who admit they made a bad call, will usually only cite the consequences to themselves and their family. If they ever come around to actual stranger empathy it will be at least another year or two later.

Religion or family trains them from an early age that X==bad.

Evolution for uncounted generations tells us to make babies. It's a super weapon to use that as the basis for "those people bad"

It takes a lot break a person away from that mindset.

It's even harder if you're raised in an area where eveyone is struggling to bootstrap.

Insecurity. They need someone to view as less than so they can seem better by comparison. It's not just religion.