I'm not sure about your "90%" figure. While many who commit suicide suffer from diagnosed or diagnosable mental illness, many do not.

Many commit suicide owing to horrible external circumstances. To turn it around and say it was caused by depression is unreasonable. And in fact, none of the 4 factors you mentioned are "mental illness."

Also, as I noted in this (unscientific) post, mental illness is not in the "TOP 4" reasons among Japanese. Having lived there for almost 2 decades, I can safely say that I do not recall one instance of hearing about a suicide owing to mental illness.

On the other hand, I did hear of suicides owing to the above external factors – too often!