You can find the decision here. Here are some important paragraphs.
His crimes are so grave that it is hard to believe that he should be released. He is a threat to public safety, so I have decided that he cannot be paroled. After spending decades behind bars, Sirhan failed to correct the problems that lead him to murder Senator Kennedy. Sirhan doesn’t have the experience to prevent him making dangerous decisions like he did in the past.
Most egregious evidence of Mr. Sirhan’s lack of insight is his changing narrative about Senator Kennedy’s assassination, and his refusal to admit responsibility. As the following examples show, Mr. Sirhan has inconsistently described his role in the assassination of Senator Kennedy, claimed shifting memory lapses, minimized his participation in the crimes, and outright denied his guilt….
His current level of risk isn’t reflected in his potential risk to commit acts of interpersonal violence, however. Sirhan is a danger to the public safety as he does not have insight. This can be seen in his refusal to answer questions.
Accept responsibility for Senator Kennedy’s assassination, the failure of him to abstain from political violence and the lack of skills necessary to deal with complex external triggers. Evidence of Mr. Sirhan’s declining physical strength is not sufficient to mitigate the threat to public security he represents, and the potential for him inciting violence against political opponents if he is allowed to be freed on parole. His release does not meet the criteria for public safety.
Given my limited knowledge of parole policies, I am unable to comment on the merits. However, I must point out that while a person cannot be sent to prison for the “risk that they may incite violence” it is possible that parole decisions regarding people who have committed or are about to commit political violence will often hinge on these matters.