Diabetes type 2 is a condition that caused by relative lack of insulin. It called relative because it might be caused by the reduction of insulin or resistance of the cell. In this case, the pancreas cell still alive but their work is not enough.
So, the treatment used would be to decrease glucose absorption, increase cell sensitivity to insulin(like metformin), or increasing the amount insulin secreted by drugs like sulphonylurea. Insulin injection will be the last choice.
In diabetes type 1, the pancreas cell is already destroyed. In this case, sulphonylurea drug won't work since there is no pancreas cell left to secrete insulin. Increasing sensitivity also won't work since there won't be any insulin. The only treatment for diabetes type 1 is insulin injection.