As I see it, it is because Mankind sets up a system of behaviors that are acceptable and not acceptable based upon ideas and beliefs that are not necessarily in their own personal best interest. For the most part, other creatures do not.
For example, hyenas and lions on the Savannah have no qualms about appropriating each other's kill for their own meal, while to humans that is stealing and immoral. Now members of a pack will make sure that their young will get fed, and give food to the alpha, and other such behaviors, but all of these can be traced to selfish ends, such as ensuring their blood line continues, protecting the pack (because the pack protects them), etc.
Now, this isn't to say other animals can't express moralistic behavior; dolphins and non-human primates have been known to do so from time to time.
At least, that's how I see it. No particular education or expertise on the subject, though. If you're turning this in for homework, you're still going to have to do your own research and find your own citations.