We assume that it is because of the righteousness of Mormon and Moroni that they were privileged to see the three Nephites. However, the three Nephites were given the discretion regarding those to whom they would show themselves, if they shall pray unto the Father in the name of Jesus they can show themselves unto whatsoever man it seemeth them good (3 Nephi 28:30). This passage implies that the three Nephites specifically requested to show themselves to Mormon and Moroni. We are left to wonder if the three Nephites weren't as thrilled to meet Mormon and Moroni as these prophets were to meet the three Nephites.