Why could the Gospel go to the whole world after Jesus' resurrection?
Because Christ had completed the work of the cross—His death, burial, and resurrection—He became the object of faith for the whole human race. Before that, His earthly ministry was almost exclusively to Israel; after His resurrection, the Gospel could go to all people, not just Israel.
Les teaches that Jesus' illustration of the seed dying and bringing forth much fruit pointed to His coming death, burial, and resurrection. Until He had experienced the work of the cross, He was not an object of faith for Gentiles, and His earthly ministry was almost exclusively to the nation of Israel. But after His death, burial, and resurrection, He became the object of faith for the whole human race, so the Gospel could now go beyond Israel to all people. Les also notes that Paul emphasizes resurrection because when Christ arose from the dead, He exhibited more power than it took to create the universe.
Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit.