Let's be honest. There's really only one such factor:
Paroling her would be politically unpopular and could possibly be used to the disadvantage of Democratic candidates, by "law and order" opponents, in future elections.
That's the reason -- the only reason -- why California governor Jerry Brown vetoed the parole board's recommendation.
She's been in jail or prison for nearly 50 years -- since 1969, when she was 20 years old.
There's good reason to believe that that her culpability in the murder of Rosemary LaBianca is somewhat mitigated by circumstance. To wit, she was effectively "brainwashed" -- recruited by the Manson "family" specifically because she was vulnerable to their approach, being a confused, drug-addled, mentally troubled teenager with an ugly family backstory (including a forced abortion).
She's been a model prisoner, never a disciplinary problem. She'll turn 67 in August.
Yes, it is true that but for the Manson "family," including Leslie Van Houten, who assisted Tex Watson in killing her, Rosemary LaBianca would either turn 86 this December or else have died in some other and almost certainly less horrific way.
It is also true that absent that single word -- "Manson" -- being associated with her crime, Van Houten would almost certainly have been paroled long ago. That may be a good thing or it may be a bad thing, but there's no doubt that the reason it's a thing at all is politics.