Because he's a coward. Because he's been programed to be a coward for the entirety of his life.
From the time we are children we are told "Don't hit!" and "Fighting is wrong." As youths we are told that "Violence never solves anything." and "It's not worth fighting over." violence is discouraged through punishment; detention, suspension and expulsion. As adults we are told "It's not worth dying over." and violence of almost any form is punishable by law, both criminal and civil. Even in some cases, self defense.
Admittedly children are poor judges of what constitutes "justifiable circumstances" and youths often have a poor sense of "appropriate response". An adult however should, and must be able to make these judgements, and to have trained them not to use violence when appropriate, is a disservice to the adult and the community.
We live under the Rule of Law almost in name only. The powerful and the rich do as they please and are only occasionally called to answer for it. More often than not the punishments are light, if any at all. And punishment for those who employ violence righteously is almost as common. Yet should we shirk our duty to our community? Should we allow ourselves and our loved ones to become victims, to save ourselves from punishment?
All that is necessary for evil to triumph is for good men to do
A man who won't die for something is not fit to live.
-Dr. Martin Luther King Jr.