Decades of some performers out of their heads on drugs or drink or both. Car crash personal lives. Stints in rehab. Lots of performers can't cope.
Susan Boyle seems to have coped well by comparison. As someone has said, there was oxygen deprivation at birth. That has caused some problems but whenever i've seen Susan Boyle on camera she has been chatty, articulate, warm,intelligent and has a great sense of humour. I'm not even a fan but a slight, disability doesn't make her incapable of coping pretty well.