There is no essential difference between a bishop and an archbishop.. as far as Our Lord not being into such, as the number of His followers expanded some form of administration became necessary. What could be more fitting than it should be those who had received the grace of Holy Orders, the charism of carrying forth the teaching Christ bequeathed to His Apostles, the Bishops, (overseers). The divine institution upon earth, the Church needs to be be administered through a hierarchy in order to preserve its unity.