It is very clear cut.
Mexicans are willing to work for less money than American citizens. It’s worse than that, though. Mexicans send their disposable income back to their families in Mexico instead of spending here with local American businesses.
30 years ago you could hire construction workers for $10 per hour. Go to the Home Depot parking lot today and you can still hire construction labor for $10 per hour, but none will be American citizens.
They have no interest in building a life here.