Why was prophecy the greatest gift when Paul wrote to the Corinthians?
Prophecy was the greatest gift because the New Testament had not yet been written. The early apostolic churches had no printed Word, so they depended on gifted men to proclaim God's truth. Paul therefore urged them to desire the gift of speaking forth the Word of God.
Les explains that when Paul wrote to the Corinthians, the New Testament was still unwritten. The Gospels would not be written until years after Paul's letters, and his own letters had not yet gone out as the Word of God. These early, primitive, apostolic churches were growing and reaching out in the pagan world without the benefit of the written Word. They had to depend on gifted men who could teach people the Pauline doctrines and not that which was still coming out of the Old Testament. Paul, by inspiration, recognized that the most important thing the local church could have was men who could proclaim the truth of God's Word without having it in print. Later, as he said in chapter 13, that gift would fade away because now we have the printed page and no longer need men gifted to proclaim the Word of God without anything to go on.