one word: ransom.
jesus, on earth, was merely a perfect human, that is to say, a sinless human.
he had to be that and nothing more, nothing less. otherwise the ransom wouldn't have been paid.
adam = perfect man
adam sinned, so sin spread to all his offspring.
jesus = perfect man
jesus died as that same perfect man, thus repurchasing or ransoming all of adam's offspring.
if jesus was anything other than a perfect sinless human, his sacrifice would have been applaudable but wouldn't have repurchased anything.