"In 2013, for the first time, more than a third of migrants whom the U.S. Border Patrol apprehended were not Mexican. The overwhelming majority of these 153,055 “other-than-Mexican” apprehended migrants came from Central America, mainly El Salvador, Guatemala, and Honduras. The number of other-than-Mexican migrant apprehensions has nearly tripled in two years; there were 54,098 in 2011. (More apprehensions most likely means more migrants overall, though the number who avoid capture is unknown.)
The wave of Central American migrants has intensified. Eight months into the U.S. government’s 2014 fiscal year, which began in October 2013, Border Patrol had already apprehended 162,751 “other-than-Mexican” migrants, exceeding the full-year total for 2013."
That's why.
http://www.wola.org/publications/mexicos_other_border