Because the Democrats picked it up and used it as an issue as well. Instead of dispelling the myths of “Bigrant Crime,” as if that would be a challenge, they took an anti-immigration stance as well. Honestly, Trump had a rare master class attack on the Dems with the whole “they’ve been in power for four years, why haven’t they already done everything,” because it called out just how shallow the Dems bending to conservative anti-immigrant policy was. It was almost certainly an accident on his part, especially since his party was the reason Dems couldn’t be as hard on immigration as they wanted, but it turned into effective messaging.