"There are other causes too like mental illness."
I'd wager that a rather large percentage of substance abusers had some sort of "mental illness" to begin with, and more or less consciously started self-medicating. Alcohol works wonders for depression and certain anxiety disorders ...For a time. After that it's a vicious circle that's extremely hard to break - and without help, near impossible.
Look at how hard it is for an otherwise healthy individual to stop smoking, then imagine having some range of mental disorders and living on the streets. Where are you going to find the resources to break out of an addiction on top of that?
Anyone who thinks it's just a matter of these people getting their priorities straight, are being seriously naive and/or ignorant.