RoboCop, a cinematic gem of the 1980s, exemplifies the attitude of many people toward Detroit: crime-infested urban ruins. So, despite the title character being [spoiler alert] a hero, the movie's setting is not depicted in such a positive light.

This is in contrast to Rocky, which shows Philadelphia in a mostly-neutral light; along with a glorious training montage that made the Philadelphia Museum of Art steps an icon and tourist attraction. Given the positive effect of Rocky on Philly tourism, the strong identification of that film with a particular filming location, demand for a statue for photography purposes, and the pre-existence of a bronze Rocky statue (which was created as a prop for Rocky III), the permanent installation of the prop statue seems natural.
Now, in Detroit, with the resurgence of civic pride and national recognition: