Physical disabilities and illnesses come about due to clear actions, and are generally recognizable as universally bad.
These sorts of things either happen near-arbitrarily (birth abnormaility, blood clot) or via an accident (falling down the stairs). After the fact, you're less able, in general.
By contrast, many mental illnesses are triggered by society, and some make no sense without a societal structure.
People go to work, become stressed at the lack of reward or meaning in their employment and get depressed. Now they have an 'illness', because they no longer want to turn up for work. Cyclical reasoning.
There are certainly many mental illnesses that aren't covered by this, but I think that when comments are made about 'declining mental health in society' - it's really just throwing diagnoses at the fact that we're becoming more cut-throat....