Because, the idea was that we were sick from the outside in so, therefore, if we were sick from the outside in, then it was thought we needed to get well from the outside. It goes back to the germ theory. If we believe that germs actually do cause disease, then let s take something from the outside that will fight the germ. The reality is that germs only infect sick or diseased people. So, you have to be sick on the inside.
Germs are merely the opportunistic vectors of disease.
They are not in and of themselves the cause of disease. Therefore, for us to be well, we need to be well on the inside. By getting well on the inside, the ability for us to fight off the germs has been increased therefore, we won t get sick. Because one person was well on the inside and the other was sick on the inside. The illustration that Jesus uses which I think fits in so perfectly with that is the parable of the sower.