This isn't too complex.
Employers by the thousands contribute to both parties, and in fact bankroll their campaigns etc. Dirt cheap labor is worth a fortune. These employers have no desire to see their low wage golden goose killed off and they'll pay big to make damn sure the borders remain lawless and wide open. It's why things like E-Verify are nothing but window dressing, voluntary not mandatory for employers. Look at your voting laws...Most are a joke designed to be compromised. And on and on and on...
There are many reasons why the Republican party has watched our borders and sovereignty totally deteriorate for 30++ years now. Even during war time, under Republican administrations.