I assume its because the housing stock is bigger, therefore there is not a need (i.e. forced) to expand your "living space" into the outdoors. Furthermore, the proximity to the water and ability to view that water from said rooftop decks also may play into this. In North Baltimore there is no way to see the harbor, so decks may not be as popular. Additionally, the proximity to water adds immense value to the home and the market forces may play a role in people's decision to add one - and add an extra, say, $10k onto their house - if they go that route.