Because God did not want what has happened since? Look what life has been like once they knew of evil! God gave them Good. He was happy to give his children good things. However, once they learned evil they knew the "difference between" good and evil.
Adam and Eve chose evil while not having a knowledge of evil. Since that day there has been a war between Christ and Satan to save souls to be saved (God) and to tempting souls to be lost (Satan)! Could it be that Christ knew what would be required if they gave in to evil?
I am not here to say that I know why, because I would not dare to presume to know the mind of God. All I know is the evil that abounds today!