Probably the same reason the translators used "soul" for man and "creatures" for the other animals God created, this led people to believe that the difference between man and animals was that man had a soul. Not so, both man and animals were said to be souls, same word. Just like lust is used to described a strong desire for something bad while desire is used to describe something good when both words mean the same thing. I think the translators would have done better to just use the same word and let us realize the difference because of the context.