If it was.murder, we will never prove it. Suicide is a convenient ruling for cops, especially in a high profile case, since it is less work for them, and it looks bad when you have a murder but no suspects, especially when it is a celebrity. And given Marilyn's association with the Kennedys, there was no doubt a lot of pressure to wrap it up quickly, and the Kennedys had the clout to make it happen. They demonstrated this when Edward Kennedy was guilty of negligent homicide but got off scott-free​ in Chappaquiddick.
Sure, Marilyn was maybe depressed, but that is a long way from killing yourself, and she could have made trouble for the Kennedys, she was rumored to to have Kennedy's mistress, which we now know Kennedy had many. The press chose to overlook Kennedy's numerous affairs, but back in the 1960's had Marilyn made her affair with Kennedy known publicly, it could have severely damaged Kennedy's political career, even perhaps ending it.
Personally, I suspect that could be a reason that there was such an effort to whitewash Kennedy's own assassination, because it the full truth came out on his assassination, then the truth of his involvement in Marilyn's death could have come out. That is why his own brother would have gone along with any cover-up, who was the Attorney General. Not only would his brother's reputation suffer, but his as well, and he had his own political ambitions.
You can bet that no one in California is interested in the truth if it will tarnish Kennedy's reputation. Any official investigation will just be a whitewash, there would be a can of worms that would be opened if there was a legitimate investigation, too many embarrassing questions to answer if the police now admitted it was murder.
It's interesting and there are reasons to think she might have been murdered, but she had also had accidental overdoses in the past and was known to have abused prescription drugs for a long time. She had taken her usual barbiturates but unusually had also been prescribed chloral hydrate? a few days before and they are not supposed to be taken together
She was a pill popping junkie with major depression issues where basically everyone who knew her knew how volatile her moods could be. Getting any RX she wanted because she was rich. No surprise, people like her don't live long.