Anti-immigrant sentiment in the US has been a thing for hundreds of years. Consider watching Scorcese’s “Gangs of New York” for a (fictionally dramatized) depiction of it in times past.

As for why mass deportations are possible today - - until the late 1800s, immigration to the US was essentially unregulated. The Chinese Exclusion Act and later systems of quotas and literacy tests introduced around the turn of the 20th century instituted the first national immigration policies.

I frankly don’t find it unfair or unreasonable that the US government’s executive branch has chosen to enforce existing immigration laws for political gain. Americans should change their immigration laws if they get upset when they’re actually enforced. If anything, the executive branch was utterly failing to enforce laws that representatives had placed and kept on the books for a long time. If you want more immigrants, make it easy and legal to receive more immigrants without tests, long wait periods, or country of origin quotas.