An individual's finances can affect the means by which they can do physical activiry, which in turn affects their ability to keep physically fit.
If, for example, an individual has a small amount of finances to their name, they may not be able to access specialized equipment that would normally be found in places such as gyms (which require a paid membership most of the time).
In short: There's only so much one can do for free and without any kind of fitness equipment.