Until 1965, we had a very small illegal immigration problem. After 1965, we had a HUGE illegal immigration problem.
I submit that the two are a study in cause and effect.
There may be some relationship between the two, but to suggest this is the single cause and effect seems a bit simplistic. The internal deterioration of Mexico is another factor. The meteoric rise of the American economy and subsequent prosperity since 1965 is another. Parallel to that, the propensity to break/ignore/generally question law/authority among our people contributes to an atmosphere of acceptance of hiring illegals.
I think there are many contributing factors.
Ending the guest worker program was the biggest piece, IMNHO.
Suppose a law were enacted forbidding you from obtaining gainful employment.
You now have two options:
1. Starve
2. Break the law
Which one would YOU pick?
Guest workers usually took their money and went back home to their families at the end of the season.
The problem is that, without a guest-worker program, each border crossing is made illegally and thus has an element of risk.
To minimize this risk, the illegal alien has to cross once and stay. That changes the dynamic considerably. In addition, the illegal alien now brings his family--and so, instead of one guest worker coming and going, we now have 2-5 illegal immigrants here on a permanent basis.