I can understand the family's frustration, they believe, as did the police and prosecutors, that he murdered their daughter and want to see justice done. Sadly, I think the bottom line is its easier, quicker and cheaper for the authorities to accept the manslaughter conviction and get this over with than to go to the time, trouble and expense of a murder trial. No matter what any of us think, nobody can tell which way a jury may go in a murder trial.
Having said that, I suspect the family now have a very good civil suit if they wanted to go that route. Remember OJ was found not guilty in his criminal trial but was deemed responsible in the civil suit. Nothing can replace their daughter, but if they choose, this man could be paying, at least financially, for a long time to come.