Remember that before the invention of air conditioning, large cities were hot, sticky, and unpleasant places to be in the Summer.

In many large, important cities, the wealthy would travel to the country to spend the summer. The English were particularly fond of this ritual, although I wrote an answer about the "town-and-country" rituals in the Republic of Venice you might be interested in.

"Town-and-Country" rituals were particularly prevalent in political capitals: families would only live in the city during the fall and winter while the head of the household participated in the legislature or the large apparatus (formal and informal) both inside and out of government. New York, however, being the economic (rather than political) epicenter of the United States, working professionals didn't have the benefit of a long legislative recess to head to the country. Although they might have limited holiday time, they might as well send the rest of their family out to the country to escape the city heat!