This article is from Lawfare, and the answer is interesting.

First off, while the current administration has tried to tie Central American migrants to MS-13, government data reveals that gang members crossing irregularly are the rare exceptions. Since the Trump administration took office, the Border Patrol has detected fewer gang members crossing irregularly than during the Obama administration. In FY2017, these detections amounted to 0.075 percent of the total number of migrants (228 MS-13 members out of 303,916 total migrants). When combined with MS-13’s rival, the Barrio 18 gang, the number rises only slightly to 0.095 percent. This is far from the “infestation” of violent gang members described by the president…

The face of migration has also changed. Back in 2000, Mexican nationals made up 98 percent of the total migrants and Central Americans (referring to Honduran, Guatemalan, and Salvadoran migrants) only one percent. Today, Central Americans make up closer to 50 percent.

A declining Mexican birth rate, a stable economy, and the U.S. border buildup have all contributed to the decrease in migration from Mexico. But as Mexican irregular migration has plummeted, Central American migration has simultaneously picked up. Until 2011, Central Americans constituted less than ten percent of total U.S.-Mexico border apprehensions, but by 2012 they constituted 25 percent, and by 2014 they numbered half of all illicit border crossers. While migration from each country within the Northern Triangle (El Salvador, Guatemala, and Honduras) has fluctuated over time, each country has sent roughly similar numbers of people in the aggregate. From FY1995 to FY2016, the U.S. Border Patrol apprehended around 500,000 citizens from each country. In other words, it’s not a coincidence that most recent news stories about migrant parents separated from their children feature families from Honduras, Guatemala, and El Salvador.

People from Honduras and El Salvador are often fleeing urban gang violence, while those from Guatemala are often fleeing rural poverty.