Then I'm sorry, those were some piss poor prosecutions. It's not just the facts but the presentation (i.e. OJ)
A couple of jurors interviewed after the cases closed made it clear that they didn't consider giving someone a job to be a criminal matter, and that the law of the land was contrary to the custom of the land in that respect.
The modern illegal immigration problem started in 1965. That also happens to be when the guest worker program ended. I submit that the two events are related.
When laws diverge significantly from what the people at large think the law should be, those laws are going to get ignored by a lot of folks. Note the abject lack of prosecutions for violations of the Roberti-Roos Act in California.