She has terminal brain cancer:

Former Charles Manson follower and convicted murderer Susan Atkins, who has brain cancer and lost her bid last year for a “compassionate release” from prison, has been denied parole again.[…]Rackauckas wrote in his letter last year that “it would be a grave miscarriage of justice to burden the citizens of Orange County, where she can enjoy the comforts of her husband, home, and mercy she did not show Sharon Tate, her unborn baby and seven others.”

Here’s a report from last year when she tried for a compassionate release: