In addition to Bye's good answer, I would like to add the following.
Uptill this point only one other person has ever ascended to heaven (from the written record) - that was Enoch. The major difference between Enoch and Moses was that Moses was resurrected from death while Enoch was translated alive.
Paul says in Romans,
Romans 5:14, 17: Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come... 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.)
Death reigned from Adam to Moses - The power of death over the human race was hitherto unbroken. Death seemed to rule with an Iron Scepter. Satan who is the father of sin and thus the originator of death apparently had that unbroken power - Once you die, no matter how true you were to God, you belong to Satan (as everyone who sins are children of satan).
But now - Wow, death's power was broken! and a captive of Satan was being led away by Michael toward heaven. If this would happen to everyone who would obey God.... Satan could not permit it.
That's why he contends when Michael came to retrieve Moses.