Why do most scholars think the last twelve verses of the Gospel of Mark (Mk. 16:9-20) were not written by Mark? Lee Strobel calls on manuscript expert Dr. Daniel Wallace to answer here. Wallace, who thinks the last 12 verses were added later, has an interesting insight:
One can easily see why scribes would want to add to Mark’s original ending, however: he wrote his gospel in such a way that would make the story open-ended. He wanted the reader to step into the sandals of the disciples and make a decision about Jesus themselves. Mark’s original ending was a brilliant maneuver on his part for he was drawing the reader into the narrative, showing that it was impossible to accept Christ in his glory unless one also accepted him in his suffering.
What does the inclusion or exclusion of verses 9-20 mean theologically? Nothing. If they are included, nothing new is taught. If they are excluded, nothing is lost because the resurrection appearances are described elsewhere.