But those are different things. victim's is a genitive s meaning 'that of the victim'. That is how we write that word, there's no other way.
For the verbs is, are etc Duo automatically generates/accepts contractions for us in many cases, but not in all. who's is actually automatically accepted for who is, but you were asking about why's, which is not automatically accepted for why is. – At least if you meant this comment: https://www.duolingo.com/comment/7963129
Possibly Duo chose not to make some combinations accepted because they're considered less standard or something, I don't know. But it would be too much work for us to add them all manually.