It's amazing how "properly dressed" everyone is compared to current standards. Almost everyone, including young adults, are dressed in a suit and tie with a hat.
I think all those trains, cars, carriages and people would’ve made a lot more noise than the audio mix suggests. The trains and cars especially should be drowning out everything else as they pass near the camera. This sounds more like the audio track of a feature film than anything captured with a microphone in the field.
Maybe there were fewer people in general, but did you notice how few private vehicles there were, and many people used transit or simply walked? How many people walk across the Brooklyn bridge today vs. driving in a car?