Why, no one knows, since he's assured us repeatedly that the literal millions of illegal aliens that have crossed at his direct behest, unvetted, unscreened, and mostly unknown, aren't a problem.
I mean, it's not like Mexico and the rest of Shitholia and Trashcanistania are sending us hundreds of thousands of terrorists, criminals, murderers, and rapists, exactly like Trump said.
Nota bene, reality is between 33 and 100 times worse than officially reported, because CBP, on its best day, catches maybe 3% of crossers, and most days, it's closer to 1%.
So adjust those numbers of criminals who succeeded in getting here upwards, commensurately.
Which leads to an inarguable truth concerning the real reason for Poopypants' sudden attention to the disaster he's created, the existence of which catastrophe he's repeatedly denied:

