A: There are generally three possible reasons given, but no one knows for sure.
1. John was a part of the Old Covenant, rather than the New Covenant.
2. John was not physically able to be with Jesus, and therefore was unable to see, hear, and personally witness such things as Jesus’ teachings, miracles, death, and resurrection.
3. John was a friend of the bridegroom rather than the bride (Jn 3:29).