Back In the day, ideas of racism and elitism were far more popular than they are now. This did not bode well when many immigrants came from families of color. So many people simply thought of them as inferior humans. There was also a popular yet unsound thought within Americans a while back that most people that were immigrants must be illegal, which only further led to discrimination towards immigrants, whether they were illegal or not.
Today, though racism and elitism are now generally shunned upon by society, Many Americans are skeptical of immigrants due to the influence of Donald Trump ( -_-)