Mass migration is a very recent phenomenon.
There is no historic precedent.
An influx of workers prepared to undercut the established workforce brings benefit to big business, government and the professional classes.
The workers are ground down with lower wages, unemployment, competition for social housing, higher rents, reduced availability of health provision, social services, etc., etc.

And anyone who complains is branded a racist by big business, government and the professional classes.
And of course, you can not expect professional people to debate with racists.