They already built some of the border wall, and it has made no impact all. Water will always find a course.
The number of people saying this is a serious or very serious problem really jumps out. The candidates have talked about insecurity, but they have failed to offer concrete proposals, and that's what the voters want.
Most Mexicans know someone who has gone to the United States, or someone who receives money from someone over there or someone who is thinking about going themselves. Seeing the marches of immigrants in the United States in Mexican media has also had a huge impact here.
He has a way of speaking that is closer to the people. He comes from the (political) left, and there is the perception that he would do more for those who are the ones who leave.