Objection 1. It would seem fitting that the disciples should have seen Him rise again, because it was their office to bear witness to the Resurrection, according to Acts 4:33: "With great power did the apostles give testimony to the Resurrection of Jesus Christ our Lord." But the surest witness of all is an eye-witness. Therefore it would have been fitting for them to see the very Resurrection of Christ.
Objection 2. Further, in order to have the certainty of faith the disciples saw Christ ascend into heaven, according to Acts 1:9: "While they looked on, He was raised up." But it was also necessary for them to have faith in the Resurrection. Therefore it seems that Christ ought to have risen in sight of the disciples.
Objection 3. Further, the raising of Lazarus was a sign of Christ's coming Resurrection. But the Lord raised up Lazarus in sight of the disciples. Consequently, it seems that Christ ought to have risen in sight of the disciples.
On the contrary, It is written (Mark 16:9): The Lord "rising early the first day of the week, appeared first to Mary Magdalen." Now Mary Magdalen did not see Him rise; but, while searching for Him in the sepulchre, she heard from the angel: "He is risen, He is not here." Therefore no one saw Him rise again.
I answer that, As the Apostle says (Romans 13:1): "Those things that are of God, are well ordered [Vulgate: 'Those that are, are ordained of God]." Now the divinely established order is this, that things above men's ken are revealed to them by angels, as Dionysius says (Coel. Hier. iv). But Christ on rising did not return to the familiar manner of life, but to a kind of immortal and God-like condition, according to Romans 6:10: "For in that He liveth, He liveth unto God." And therefore it was fitting for Christ's Resurrection not to be witnessed by men directly, but to be proclaimed to them by angels. Accordingly, Hilary (Comment. Matth. cap. ult.) says: "An angel is therefore the first herald of the Resurrection, that it might be declared out of obedience to the Father's will."
Reply to Objection 1. The apostles were able to testify to the Resurrection even by sight, because from the testimony of their own eyes they saw Christ alive, whom they had known to be dead. But just as man comes from the hearing of faith to the beatific vision, so did men come to the sight of the risen Christ through the message already received from angels.
Reply to Objection 2. Christ's Ascension as to its term wherefrom, was not above men's common knowledge, but only as to its term whereunto. Consequently, the disciples were able to behold Christ's Ascension as to the term wherefrom, that is, according as He was uplifted from the earth; but they did not behold Him as to the term whereunto, because they did not see how He was received into heaven. But Christ's Resurrection transcended common knowledge as to the term wherefrom, according as His soul returned from hell and His body from the closed sepulchre; and likewise as to the term whereunto, according as He attained to the life of glory. Consequently, the Resurrection ought not to be accomplished so as to be seen by man.
Reply to Objection 3. Lazarus was raised so that he returned to the same life as before, which life is not beyond man's common ken. Consequently, there is no parity.
Source: Summa Theologica (New Advent)