A diseased pancreas is not, in itself, a death sentence. It is the collateral damage to other organs that makes pancreatic cancer so dangerous.
The pancreas is a six-inch-long organ that secretes digestive enzymes and hormones such as insulin. It continues to function semi-normally even with a tumor growing inside. For this reason, symptoms of pancreatic cancer which include jaundice (yellow skin caused by the accumulation of toxins), abdominal and back pain, nausea and weight loss usually don't set in until advanced stages of the disease. Consequently, diagnosis often comes late.
At that point, the tumors are significant in size and may encapsulate major veins and arteries. Furthermore, because the pancreas is located at a junction of several organs, cancerous tissue often spreads to the liver, gallbladder or intestines early on. Surgical removal of widespread tumors isn't viable.