Despite the the title itself not being news, there is an insight in this (short) article and it IS worth actually reading IMHO.
I wonder if anyone is doing a study into the particulars on the three groups to try and find common characteristics to guide future prescription? The cynic in me assumes not as that would reduce demand so why would a manufacturer fund such a study? But perhaps I am over cynical.
Speculating wildly for a moment, think that misdiagnosis is one likely reason. Maybe anti-depressants work on depression (the disease), but do not work on (say) CPD (where depression is just a symptom). Of course actually correctly diagnosing people will be time consuming and expensive. And I personally am not convinced we actually have a good grip on what diseases exist in mental health but that is another whole discussion...