I would have to speculate then that your father is one of the few who succumb peacefully and naturally. I am no expert and don't have statistics but do have knowledge of many many friends and family who have suffered through or died as a result of cancer. ALL of them suffered greatly through treatments with pain and/or illness from radiation and chemotherapy and ALL who died did so while being heavily medicated due to their discomfort.
I worked on a medical floor as a nurse 20 years ago. This was when Hospice was relatively new so terminal patients came to the hospital to die.
I have seen many, many, many cancer patients die. I have never seen a patient die as you relate your fathers death. Never.
Death is not an easy part of life to go through. Your father had to have medication to ease his pain.
Unless he has supernatural help from above.