A few things.

1. Fear of the “other,” with the “other” being people who don’t look like they do (with them usually being insulated within their non ethnically diverse social groups) and the fact that they’ve repeatedly seen these “other” people associated with traits that are undesirable through media.
2. False history, primarily a belief that stems from the prior point, with the assumption that white people are more “moral” or “civilized,” and that the nation was better before things like racial integration, something that they’ve repeatedly been made to believe through, again, heavily biased media, and inaccurate historical portrayals of different cultures.
3. Misdirected blame for negative factors in our society, primarily by right wing media and talking heads like Trump, that casts blame for issues specifically on certain racial groups. (i.e. it’s not that we don’t fund our welfare programs enough, it’s that “they are taking welfare payments and being lazy!”)
4. “Efficiency,” in the sense that they believe having less of these “freeloaders” will allow us to broadly spend more of our money/time/resources on what “matters” (white people) without understanding things like, y’know, the fact immigrants provide more in taxes than the average American overall since they don’t receive the same amount of benefits back from things like our welfare system.
5. Race-based nationalism that leads them to believe that they are the only people that are “supposed” to live here in America, or the only ones that “deserve” it. If you look at how they often classify immigration, or even black people simply moving in to traditionally white areas as an “invasion,” you can see how they don’t exactly view these people as members of their own nation, but rather, some outside group.

Gives them someone to blame

They want a white Christian ethnostate