Why do schizophrenics have a higher rate of suicide? what causes them to commit suicide?

Lots of reasons: Lots of reasons. Schizophrenia can cause terrible emotional pain from which a person may want to escape. They may have hallucinations or delusions that they are being attacked or persecuted. They may be afraid they will harm someone else. They may see no hope for their future. They may be unable to hold a job or maintain a home. Some end up homeless. All of these things can play a role.

Part of the disorder: Schizophrenics have a higher rate of mortality due to the nature of the disorder. The causes of suicidal behavior are not as predictable or consistent and are likely to be unique to the individuals and their thought processes.

Hallucinations: Imagine having hallucinations thousands of times more terrifying than any horror movie ever made, and not being able to control those hallucinations which can occur at any time, and for hours. The sufferer is warn down by sleep loss and prolonged exposure to deeply visceral terror from which there appears to be no escape handy. This is what outs them at suicidal risk.