Because Obama's DACA Executive Order has a two-year moratorium on the deportation of anybody who had entered the country as a minor.
Of course, that was supposed to be minors already in-country, not minors yet to come.
DACA Executive Order , that’s the name of the order that all the invaders know about, for which we are clueless. It is probably stamped on those brochures from Homeland Security, that they have been clutching on their way to the processing centers, er to the government goodies orientation .