The answer to that question can be found in the Gospels themselves.
The Gospels offer a stark contrast between St. Peter and St. John. Peter was the short-tempered, flawed human being who would deny Christ three times during the night before the crucifixion, while John "the beloved" was the favorite disciple of Christ. John was the only one of the twelve apostles who was not afraid to follow Jesus to Calvary, and consequently it was John to whom Our Lord entrusted His mother. John was also the only apostle whose love for Jesus Christ was so unquestioned during his life that he didn't have to die a martyr's death.
And yet Jesus selected Peter as the rock upon which He would build His Church.
There is a lesson to be learned here -- this was not just an accident of history. To be an effective shepherd of men it takes more than just ardent devotion to Christ. In fact, for some modern "shepherds" the application of this devotion may even be an obstacle.