How does the diathesis stress model explain schizophrenia?
Schizophrenics must have both a biological vulnerability and and stressful situation to trigger their schizophrenia.
According to the diathesis-stress model you need both a vulnerability and a stressor (stressful situation) to develop a mental disorder.
In the case of schizophrenia, you would need a biological predisposition, as schizophrenia is at least part based in genetics, and a stressful situation, such as a change of surroundings or job, chronic debt or a divorce, to trigger it.
Just one factor or the other is not enough to develop a condition.