Hi Roy, I could be wrong but I think they were denied the tree of life because they sinned,they disobeyed Gods command to not eat of the fruit. So if we disobey God, if we sin all we have to look forward to is death, for the wages of sin IS death. But if we obey God and overcome the world as Jesus did, now, today, then we will have access to the tree of life in the first resurrection, which is Jesus.
Rev 2:7 He that hath an ear, let him hear what the Spirit saith unto the churches; To him that overcometh will I give to eat of the tree of life, which is in the midst of the paradise of God.
Rev 22:14 Blessed are they that do his commandments, that they may have right to the tree of life, and may enter in through the gates into the city.
Obviously Adam and Eve did not obey God, they sinned and had to die. I think their physical death is similar to our spiritual death if we continue to sin, could be wrong though.
Gen 2:17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
Rom 6:23 For the wages of sin is death; but the gift of God is eternal life through Jesus Christ (tree of life) our Lord.
I sincerely hope this helps you my brother, because you helped me by asking and I have thought about this many times, and only now do I understand it a little better.
Peace, G.Driggs