FIP, or Fielding Independent Pitching, attempts to isolate a pitcher’s performance based only on events they can control, such as strikeouts, walks, home runs and hit by pitch. We say they can control these events, because the outcomes are not influenced by anything outside of the match up between batter and pitcher, contrary to balls in play, which are influenced by a multitude of factors. On balls in play, pitchers are reliant on their defense and luck regarding hit sequencing and batted ball placement.
Using ERA, a pitcher who gave up a run on a sequence of fluky hits and misplays by his defense would see his ERA rise due to factors that had almost nothing to do with his performance.