Empty Cross, Empty Tomb

On Easter Sunday, we celebrate Jesus’ resurrection. The early church primarily preached two events: the death and resurrection of Jesus, the empty cross and the empty tomb. Paul echoes that in 1 Corinthians 15:3-5: Jesus died (and was buried as proof) and he was raised (and appeared to many as proof). So the empty cross and empty tomb go hand-in-hand. Why is the cross the symbol of Christianity? Why does the Gospel seem to focus on the cross? 
Scripture: 1 Corinthians 15:1-20