https://www.cnn.com/2020/03/26/us/new-york-coronavirus-explainer/index.html New York is the epicenter for now, but Covid-19 will not stop there.
The first and most obvious explanation for the severity of the area's outbreak is that New York is the largest and most densely populated city in the US, and coronavirus tends to spread in dense places.
This is same problem for any densely populated cities. Even with self isolation, the residents has to leave their apartment to pickup packages, go to the laundry room, use the elevators. Some New Yorker who has common areas such as courtyard use it for running, walking and can run into each other. Respiratory droplet can survive in the air for hours.
Some apartments sent guidelines to use the common areas, that laundry rooms and elevators are allowed for only one person(family) at a time. What if there is this service where the residents can use an app or text message to request the use of the common spots anonymously (for privacy), so that they will not waste trip to the common area and potentially avoid infection. If the common area is occupied, the request will be waitlisted and notified when it is their turn.