Why is it that instances of mental health issues are becoming more prevalent?

Huge question: Nearly 20% of us adults 18 and older have some kind of mental illness. There are many hypotheses about why this is increasing, including loss of extended families and social isolation; environmental toxins; substance abuse (both an illness & risk factor); trauma and abuse (which can be both mental health issue & a risk factor for further illness); poverty leading to not getting help early, etc.