He had to, because it was people constantly bringing up Chicago as if that's the only city where murders occur. It's become a right wing talking point on Faux News.
It's a silly argument, yet idiots keep repeating it, because that's what the sheep were fed. Then they keep repeating it as if it's some brilliant and original point. You can't care about Sandy Hook if you didn't care about the all the kids poisoned by their lunches in India.
Because Zimmerman said so. He had absolutely no reason to lie, and has proven to be a noble and honest man.