It’s possible that Myrtle wasn’t able to fully cooperate with an investigation.
While the other clues (petrification, spiders fleeing before it, it being Slytherin’s monster) are helpful, Myrtle’s testimony is incredibly helpful in identifying the monster and the entrance to the Chamber. I don’t think there’s any way you could find the latter without Myrtle’s help.
So why didn’t Myrtle tell somebody what she’s seen?
In the immediate aftermath, I don’t think it would have been easy/possible to talk to Myrtle. By the time she was in a fit state to talk, most people would think the Chamber of Secrets was a closed case. And she doesn’t seem like much of a people person – I can’t imagine her volunteering the information willingly to Dumbledore.
She made herself an unreliable witness
After her death, Myrtle left the castle and made a nuisance of herself:
“Olive Hornby came into the bathroom — ‘Are you in here again, sulking, Myrtle?’ she said, ‘because Professor Dippet asked me to look for you —’ And then she saw my body… ooooh, she didn’t forget it until her dying day, I made sure of that… followed her around and reminded her, I did. I remember at her brother’s wedding —” […] “— and then, of course, she went to the Ministry of Magic to stop me stalking her, so I had to come back here and live in my toilet.”
— Goblet of Fire, chapter 25 (The Egg and the Eye)
This won’t ingratiate herself to the Ministry, who do a lot of the investigations into dark magic. Consider also:
- They think this is a closed case (they’ve caught the person they believe responsible)
- The attacks have stopped
- Myrtle is a Muggle-born, so there’s a limited pressure on them to do thorough follow-up in the case (imagine if, instead, say, a Malfoy were to be killed at Hogwarts)
And I can’t imagine they were rushing to talk to her. And it sounds like she’s no fan of the Ministry either, so for most of the intervening period, I don’t think either of them wanted to talk.
Myrtle is likely suffering from PTSD or similar
- Being killed is a very traumatic experience. And Myrtle was only 14 when it happened – as we get older, we come to terms with our mortality (for example, through the deaths of people who are close to us) – but she’s young enough that this may be her first real experience of death.
- She had self-esteem issues before her death. She was being bullied at school for her appearance, among other things.
- She probably can’t see her parents – they’re Muggles, who won’t be used to ghosts and may not be allowed to even see her.
Even years later, it’s implied she still has depression and contemplates suicide:
“Peeves upset me so much I came in here and tried to kill myself. Then, of course, I remembered that I’m — that I’m —”
“Already dead,” said Ron helpfully.
— Chamber of Secrets, chapter 9 (The Writing on the Wall)
So if you went to talk to her – as a friend, headmaster or Ministry official – I wonder whether she’d be in a fit state to talk to you. Would she be willing, or able, to recount specific details of the night she died? There’s no evidence of counselling services in the magical world, so it’s possible she was left to just stew on her own for fifty years.
Myrtle has a soft spot for Harry
It’s implied that Myrtle has a crush on Harry – she asks him to visit her in the toilet, she follows him around, invites him to share her bathroom if he dies in the Chamber. Although it takes a while for her to warm to him, she does seem pleased to see him when he comes to visit:
Moaning Myrtle was sitting on the tank of the end toilet.
“Oh, it’s you,” she said when she saw Harry. “What do you want this time?”
“To ask you how you died,” said Harry.
Myrtle’s whole aspect changed at once. She looked as though she had never been asked such a flattering question.
— Chamber of Secrets, chapter 16 (The Chamber of Secrets)
She goes on to explain the exact circumstances of her death, down to the faulty tap in the bathroom she haunts. She’s got to know Ron and Harry through their illicit activities that year, and I wonder if that makes her more willing to talk to them. She defaults to assuming everybody hates her or talks about her behind her back – only when she gets to know them does she trust them enough to talk about her death.