Because cities don't have many trees, duh, and city life is nasty, brutish and short

 Healthy Acres

Healthy Acres

Harvard spends a lot of time and money to discover that moving to a place with lots of trees increases longevity. 

Living in the leafy suburbs or the peace of the countryside may invite disdain from inner-city urbanites but it might just save your life, according to Harvard University.
People whose homes are surrounded by the most greenery are 13 per cent less likely to die of cancer. Their risk of dying from respiratory disease also drops by 34 per cent, the biggest ever study into green spaces and health has shown.
“We were surprised to observe such strong associations between increased exposure to greenness and lower mortality rates”
Overall mortality was 12 per cent less for people who had the most greenery within 250 metres of their homes during the eight year follow-up period.
It is thought that being surrounded by vegetation improves mental health and lowers depression. It also allows people to get out and about more, giving more opportunities for exercise and social engagement, both of which are known to be protective against disease. The lack of air pollution in green areas also plays an important role.
Nevertheless, the researchers said they were not expecting the effect size to be so startling. 
"We were surprised to observe such strong associations between increased exposure to greenness and lower mortality rates," said Peter James, research associate in the Harvard Chan School Department of Epidemiology.
"We were even more surprised to find evidence that a large proportion of the benefit from high levels of vegetation seems to be connected with improved mental health.

I'll leave the science to the scientists, but I do know that when I leave Maine and cross the Piscataqua Bridge heading southwest, my blood pressure rises and my serenity drops with each mile I draw closer to New York.