Because when he got out of the car, after struggling for his weapon and being pummeled by the big thug, he told the thug to stop and then the big guy, in the words of witnesses, "bum charged," the officer.
Knowing how big and strong the guy was, he fired in fear of his life.
End of story...end of a criminal thug.
If you're Brown and you've already, according to the account, been shot once in the arm in the struggle for the gun and then have gotten away why would you stop, turn around, and "bum charge" the officer who still had the gun?