"The record indicates that Davis fully embraced and championed the family's distorted values and goals, and was willing to protect the family's interests at all costs," the decision said.
Davis would have been only the second Manson-related murder defendant to be granted parole since the killing spree began in 1969.
Davis was not involved in the notorious Sharon Tate-LaBianca killings but was convicted with Manson and others in the murders of musician Gary Hinman and stuntman ranch hand Donald "Shorty" Shea.
Manson was a direct participant in both killings, according to witnesses.
Steve Grogan, another participant in those murders, was released in 1985 after he led police to where the bodies were buried in the San Fernando Valley.
Beckman noted that Grogan, a central figure in the killings, has lived as an upstanding citizen for 27 years with no problems since his release.
Davis was 30 when he was sentenced to life in prison in 1972 in the case, which was a postscript to Manson's notorious reign as leader of the murderous communal cult.
Davis long maintained that he was a bystander in the killing of the two men. But in recent years, he has acknowledged his shared responsibility. He said his presence may have emboldened others to take action because he was an elder of the group.
Brown said Davis' refusal to fully acknowledge his responsibility for the killings was central to his decision.
"I do not believe that Davis was just a reluctant follower who passively went along with the violence," he said. "Davis was older, more experienced, he knew what the Manson Family was capable of, and he knowingly and willingly took part in these crimes."
Davis became a born-again Christian in prison and ministered to other inmates, married a woman he met through the prison ministry, and has a grown daughter. The couple recently divorced.
Davis also earned a master's degree and a doctorate in philosophy of religion.
Brown commended him for his self-help efforts but said the work was outweighed by other factors.
Manson and three of his followers, Leslie Van Houten, Patricia Krenwinkel and Charles "Tex" Watson, remain in prison for life in the Tate killings. Their co-defendant, Susan Atkins, died of cancer behind bars in 2009.