Because the way to get them on board with the migration bill is to ramp up the police state at our southern border. They revel in the ideal of jackbooted thugs harassing people and demanding to see their papers.
In general, I think the whole idea presented in the immigration bill is a bad one. It creates a class of sub-citzen, something I'm in complete disagreement with. And getting Republicans on board with that bad idea was accomplished by adding more bad ideas to the bill. I'd much rather see our immigration system changed to allow for more legal immigrants and the borders opened up to allow for free travel. This bill, while called "amnesty" is actually quite punitive to the people who are here working without having jumped through all the government paperwork hoops. That's not progress at all.