Because it is creating a problem that doesn't exist in order to look like a big shot for "solving" that problem. Illegal immigration is down, and has been for quite some time. Quietly, while none of the people screaming hysterically on either side were paying attention, the problem was whittled away at (apparently competent management and consistent policies were a big help), helped enormously by a bad economy that has never gotten much better than "meh" for people at the low end, despite all the talk of a recovery.
Nationally, illegal immigration is a minor problem. In some areas, it is much more so. But that's just geography. Of course the states right on the border will deal with the bulk of illegal immigration, that's where the border is. Port cities are naturally in the same boat. But the fact remains that overall, illegal immigration is at the lowest point since the early '90s. But Trump uses it to whip up racially based fury among his overwhelmingly white supporters. And yes, it is a race thing. In fact, it's classic, right out of the fascists' playbook: Take a group that considers itself put-upon or wronged in some way, and tell them that all of their problems are the result of Those People. It is especially effective when the target audience held a great deal of power under the old order, but have felt themselves marginalized as progress left them behind. Sound familiar? It's Fascism 101, kids.
Now, I don't mean to suggest that Trump is a fascist, at least not a deliberate one. I do not believe that he is motivated by some overarching political belief, like a true fascist. I don't think he has an agenda, or at least not one that is more detailed than "TRUMP!" He is just a narcissist celebrity who fell ass-backwards into the zeitgeist. And it's not hard to see why he is appealing. He could win. Indeed, I have a feeling he will win, at least if things remain on their current trajectory. Like I said, we usually get the president we deserve.