Even if there is usually garbage in the area (and I believe in the photos of the bag in the location it was found you can see bits of trash in the background), the bag was mostly on the sidewalk. Aside from trash day, where there might be garbage bags on the street or sidewalk, most of the trash in the city tends to be concentrated in the gutters rather than on the sidewalk itself, even in lower Manhattan where I live.
Even where I live, it is extremely unusual to see large items discarded on the sidewalk aside from on trash day. I can count on one hand the number of times I’ve seen large items seemingly abandoned on the sidewalk outside of garbage day, and I’ve lived here for five years. So I would imagine a duffel bag on the sidewalk under a streetlight would seem unusual.