In Genesis chapter two God places the burden of death on man's sin(disobedience of God).
Genesis 2:17 KJV
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.
When Jesus died on the cross all of our sins were given to him and his death paid the penalty for our transgressions.
Since Jesus suffered death for us why do Christians who have given their sins to Jesus on the cross still die?
1st John 2:2 KJV
And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world.
Galatians 1:4 KJV
Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father:
Romans chapter 5 KJV
12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
17 For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)