Because your mind heavily affects the environment in which your cells live through hormonal and other system regulation (epigenetics). A mind that is constantly stressed (for example) causes your body to be in a different hormonal state than a mind that is relaxed. This in turn can (perhaps) lead to or sustain cell dysfunction (poor immune system response, etc). Mind and body are a system. This is not woo woo, this is documented but poorly understood (placebo effect for example).

Also, none of the above implies that the mind is the cause of all disease.