Fax is still widely used in the American healthcare system for the same reason.

It's social reasoning rather than technical reasoning, combined with being excessively liability conscious and catering to the lowest common denominator.

If you communicate confidential data by email, or really anything that's electronic and remotely accessible, it's only a matter of time until someone with poor judgment opens up their work laptop with confidential data plastered all over the screen while sitting in the middle seat on an airplane.