You're missing the point that the Mexicans referred to, ". . . request my help to manage migration issues from other nations . . . "  

Mexicans made up 52% of all unauthorized immigrants in 2014, though their numbers have been declining in recent years. Meanwhile, the number of unauthorized immigrants from nations other than Mexico grew by 325,000 since 2009, to an estimated 5.3 million in 2014. Populations went up most for unauthorized immigrants from Asia and Central America, but the number also ticked up for those from sub-Saharan Africa. Increases in the number of unauthorized immigrants from other countries mostly offset the decline in the number from Mexico.            Pew Research Centre.  

Many of these illegal immigrants flow through Mexico, so having Mexico on-side with the USA in controlling same is imperative.  Mexico is not without power in this game.