1/ If a blade snaps on a normal engine, the metal casing catches it. With an open fan, that broken blade turns into shrapnel flying straight toward the passengers, fuel tanks, etc.
2/ The blades would be huge. You can’t just hang them under the wing of a normal Boeing 737 or Airbus A320.
3/ The metal casing on a normal engine acts like a giant muffler. Open rotors are loud.
4/ Open blades have no protection from bird strikes, hailstorms, or runway debris.