The problem is that, like the Democrats, the Republicans' campaigns are financed by large corporations, that want low-wage immigrant labor, both legal immigrants and illegal immigrants. And as I said, these are globalists pulling the political strings through campaign finance, who ultimately want to crush U.S. sovereignty and national identity. Was the example of Quebec not clear enough for you? In the short term, passive Republicans accepting this don't see the long-term harm. Either that, or they are more treasonous than I give them credit for.
The establishment Republicans have been soft for decades on border enforcement because those funding their campaigns want it soft. But Pat Buchanan and the Tea Party/"freedom caucus" wing of the Republicans have always pushed for stricter immigration and border enforcement. Thanks to Trump and grassroots Americans demanding it, that is gradually happening now. Even fairly swiftly by government/bureaucratic standards, thanks to direct pressure of Trump.