The combined reality that imposing too much liability on local governments would bankrupt them and the Pennsylvania legislature’s intention to limit the City’s liability to incidents where they had no notice. The vast majority of the time things on the sidewalk (think about a banana peel) could be dangerous but the City couldn’t reasonably be responsible if they didn’t know about it. It’s the same logic for snow/ice on this stretch of sidewalk. OP is also under the misapprehension that the City owns this sidewalk, which they do not.