Explain why insulin injections are not the course of treatment for all diabetics

Because during the early stages of type 2 diabetes, the main defect is insulin resistance (that is, glucose cannot get into the cell although there are sufficient insulin production by the pancreas.) What is needed is medications (e.g. metformin) that allow/facilitate the intake of glucose into cells.
There isn't any need for insulin since the pancreas is functioning normal. It's the receptors (glut receptors) causing the problem, that allow glucose into the cells