Because when a doctor does it, it sends a message to society that if you are ever in the dark deep hole of depression or if you've had a traumatic life event just happen to you that suicide is an acceptable way out of it instead of encouraging them to work towards a light in their life.
I'm the person that tells the person on the bridge not to jump. You tell them you will push them if it makes it easier.
For the most part I'd be ok with it if I held the confidence that it would stay to just terminally ill patients. The Neatherlands and Belgium have shown otherwise by offing depressed people, autistic people, mentally ill people.