It's actually pretty simple, if you think about it. Mexican produce is washed and cleaned by Mexican water. Although Mexicans have developed a tolerance for the water, most Americans will get sick if they drank it. These products that have been cleaned with tainted water eventually make it to grocery stores in the United States.
This administration has an obsession with non-American products and labor. Even though it's very clear that these products are making us sick, they'll never really give it enough attention or thoroughness to cause any disruption in the flow of Mexican products. Surely, they'll never do anything to jeopardize the relationship.
We all need to make sure we have ample supply of Pepto-Bismol on hand.