why is father mark, a priest with little experience, given a difficult assignment like kingdom village?

Mark is given his assignment for a specific reason. What we know, that Father Mark doesn't know.... is that he is dying and only has a few years to live. The Bishop gives Mark this assignment because he chooses somewhere for Mark that he would want to go if he were facing a similar challenge. He wants Mark to learn the things in Kingcome that will benefit him most, the things that will assist him in overcoming his own challenges.