The reason for the immunosuppressants is not the same as for a classic organ transplant. For a normal organ transplant, you need immunosuppressants because otherwise the body will recognise the new organ as being a foreign body that it doesn't recognise as self. For a stem cell transplant, the immune system has already gone around destroying every pancreatic beta cell it can find. The immune system is primed, and it absolutely recognises beta cells as something to destroy. The end goal is to encapsulate the inserted beta cells in a membrane that allows glucose in and insulin out, but keeps the immune system away.