Because I think the Mexican President wants to control immigration as much as he can. His people are flocking to come to the U.S. and I think he wants to help stop that. Can't say as I blame him when the drug cartels pretty much run his country.
Also, with all the Jihadi's coming to Mexico because of the almost open border, I'm sure that's one thing they talked about. If ever a suitcase nuke gets in this country, it will come in over that border.