Plus there’s the fact of The Bystander Effect: people are really, really loth to get involved in something, especially if there’s nothing they can do. Yes, seeing someone fall from a building is scary and traumatic, but most people would be a) paralyzed by fear, b) calling the cops/paramedics (NOT running toward the person) or even c) running away. Strangers and bystanders are very unlikely to actually involve themselves unless they know how to help (paramedics) or know the person (Sherlock’s homeless network/former clients) or know what to expect (again, Sherlock’s homeless network or former clients).
And yes, the fact that the uniformed nurses did NOT go with the paramedics (because they were probably costumes) but two suits did? Weird and very suspect. Unless the suits were just doctors on their break, which is possible given that they both have badges hanging around their necks … but again, why would two or three nurses stay behind to help John if they could be SAVING A PATIENT? John isn’t in any immediate danger. Three nurses do not need to be there to care for him.
Also, if you watch their behavior, all of them sort of cluster around/make a daisy chain. Observe:
Strangers don’t touch each other like that. No group of random people in their right mind are going to cluster like that, especially because, again, Bystander Effect, the people that can’t help aren’t likely to go dashing over like that. For example, look at the beginning of The Great Game: the people are sort of scattered and don’t touch each other and certainly don’t crowd up like that.