Genesis 3:23 therefore the LORD God sent him out of the garden of Eden to till the ground from which he was taken.
24 So He drove out the man; and He placed cherubim at the east of the garden of Eden, and a flaming sword which turned every way, to guard the way to the tree of life.
Why did God not allow them back into the garden? His love and mercy. See, some believe, if they would have eaten of the tree of life at this point, after their fallen state, they would be perpetually fallen. God had a plan in place to redeem fallen man. It would cost God His only Son. He willingly accepted the redemption price for fallen mankind to be restored. He navigates the channels of life for us if we allow Him to. The end will always be abundant life in Him.