It is a political wedge issue because it generates fear. Anything that causes people to be fearful is going to be exploited, and amplified, by political parties in whatever way they feel would benefit them.

It's a handle they can pull, so they pull it. It can be xenophobia, it can be "look at what they're doing in country X, what if they did that here?" It can be fear of loosing jobs, fear of wildfires, sea-level change, and it can be fear of getting sick.