Most likely because when the general public sees "3D" + "Universal Studios Hollywood" nowadays, they probably groan, roll their eyes, and say "another one?"
Trust me...it happens. I love USH but I know there's a large percentage of visitors who are tired of 3D stuff. I've been to the park with out of town tourists and while they like the park, they kept saying "it felt like we were just sitting in front of screens all day, it got old"