Because the cross of Christ cannot be divorced from His empty tomb. It is the death, burial, and resurrection of Christ which is at the heart of the Gospel. Christians must proclaim Christ and Him crucified, and at the same time they must proclaim that He destroyed the power of death through both His crucifixion and His resurrection.
ACTS 2: THE GOSPEL IS THE DEATH, BURIAL, AND RESURRECTION OF CHRIST
30 Therefore [David] being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;
31 He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption.
32 This Jesus hath God raised up, whereof we all are witnesses.
33 Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear.
34 For David is not ascended into the heavens: but he saith himself, The LORD said unto my Lord, Sit thou on my right hand,
35 Until I make thy foes thy footstool.
36 Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.