Why is it that not all type 1 diabetics are offered pancreas transplants?

Many reasons: The main reason is that one typically needs to be on anti-rejection medications with any transplant, including islet cell transplants. These meds carry significant side effects and possible bad long term effects. That is why it is typically done when one needs a kidney transplant as well. The other reason is because there are not enough donors of pancreas compared to the number of type 1 diab.

Pancreas transplant: Many type 1 diabetics have other medical conditions such as heart disease and they would not safely tolerate the operation required for a double transplant.

Not enough donors: The pancreas is a very delicate organ, and because it contains digestive enzymes, begins to auto digest soon after death. Each person only has one pancreas, so there cannot be living donors; a pancreas must be harvested from someone who has died. Also, many transplants fail. Somehow they seem to do better when a kidney is transplanted too.