It's mostly, and probably in Williams' case, due to abnormalities in the brain. There are situational cases that cause depression all the time. But when someone who seems to have it all dies by his or her own hand, my guess would be that there's some faulty brain chemistry involved. Williams did have situational reasons to be depressed, it turns out, but those things alone wouldn't cause a rational person to commit suicide.
My brain chemistry is apparently normal thanks to my meds. I lead a somewhat stressful lifestyle. I work 50-55 hours a week and take three classes a semester at college. I've been doing that year round since May of 2012. I am also not in the best place financially, having to take on debt from school and a condo that won't sell. That stuff weighs on me, but I'm not really depressed and I'm certainly not suicidal. But if I was prone to depression and I was doing those things, the pressure might weigh much more heavily on me if I wasn't in treatment.
Healthy people do not commit suicide, and having a biological problem in your brain is not restricted to ordinary people. An illness like that can take anyone down just like cancer and diabetes.
I agree with you in that hopefully something good will come of Robin's death and more people who need treatment will get it now.