The same thing that has prevented self driving cars from taking the roads, i.e. that errors can be fatal. That and doctors are better unionised and won't easily

The same thing that has prevented self driving cars from taking the roads, i.e. that errors can be fatal. That and doctors are better unionised and won't easily let a machine practice medicine. There are all kinds of laws which necessitate the presence of a licensed medical professional in various steps of treatment. Software has neither of these cushions.