The major cause of this finding is the fact that it is likely that married people are under less stress than single people.
Married people are likely to be under less stress because they have a built in support system and less uncertainty in their lives. People who are not married do not necessarily have people who they can always turn to for comfort and affection. Married people do. This is likely to allow married people to relieve the stresses that they feel on a much more consistent basis than single people can. These lower levels of stress are likely to lead to better mental and physical health for married people.