Ttyjukl

In the movie, Newt reveals that the beast attacking New York was actually an Obscurus, which he explains is

a burst of dark power that manifests in witch or wizard that represses their magical power.

Now, he goes on to explain that

The oldest known Obscurial was 10 years old, but they're usually younger.

If this is true, why then

did Harry Potter or Hermione Granger not develop an Obscurus? They were not even aware magic existed until they were 11. My only thought would be that they didn't actively repress their magic, just didn't know what it was.

I don't know if there was any more detail given in a companion novel, Pottermore or the book itself, but on the wiki it made no mention of details other than was given in the movie. Are there any additional rules that would explain why this doesn't happen more often?

Neither of them were suppressing their magical power

Newt is quite clear about this in the script; an Obscurus only develops when magic is intentionally suppressed:

Newt: Before wizards went underground, when we were still being hunted by Muggles, young wizards and witches sometimes tried to suppress their magic to avoid persecution. Instead of learning to harness or to control their powers, they developed what was called an Obscurus.

^{Fantastic Beasts and Where to Find Them (2016)}

The early chapters of Philosopher's Stone give us some insight into Harry's pre-Hogwarts childhood, and it's revealed that he had incidents of accidental magic even before turning eleven:

Once, Aunt Petunia, tired of Harry coming back from the barbers looking as though he hadn't been at all, had taken a pair of kitchen scissors and cut his hair so short he was almost bald except for his bangs, which she left "to hide that horrible scar." Dudley had laughed himself silly at Harry, who spent a sleepless night imagining school the next day, where he was already laughed at for his baggy clothes and taped glasses. Next morning, however, he had gotten up to find his hair exactly as it had been before Aunt Petunia had sheared it off He had been given a week in his cupboard for this, even though he had tried to explain that he couldn't explain how it had grown back so quickly.

Another time, Aunt Petunia had been trying to force him into a revolting old sweater of Dudley's (brown with orange puff balls) -- The harder she tried to pull it over his head, the smaller it seemed to become, until finally it might have fitted a hand puppet, but certainly wouldn't fit Harry. Aunt Petunia had decided it must have shrunk in the wash and, to his great relief, Harry wasn't punished.

On the other hand, he'd gotten into terrible trouble for being found on
the roof of the school kitchens. Dudley's gang had been chasing him as
usual when, as much to Harry's surprise as anyone else's, there he was
sitting on the chimney.

^{Harry Potter and the Philosopher's Stone Chapter 2: "The Vanishing Glass"}

Although we don't know much about Hermione's childhood, presumably she had similar experiences (though without the flagrant child abuse, I should think).

Interestingly, Harry would seem to be the perfect candidate for becoming an Obscurus; since he was quite literally abused for his magical outbursts, it seems logical that he would have tried to suppress them. Although I don't have any evidence for this, I suspect that his magical ignorance is actually what protected him; since he didn't have any notion of what was causing his outbursts, he didn't know how to suppress them.

But, regardless of quite how, it's clear that he spectacularly failed at supressing his magical powers; hence, no Obscurus.

This question is answered by Rowling in the FAQ of her new website.

Why didn’t Harry Potter develop an Obscurus?

An Obscurus is developed under very specific conditions: trauma associated with the use of magic, internalized hatred of one’s own magic and a conscious attempt to suppress it.

The Dursleys were too frightened of magic ever to acknowledge its existence to Harry. While Vernon and Petunia had a confused hope that if they were nasty enough to Harry his strange abilities might somehow evaporate, they never taught him to be ashamed or afraid of magic. Even when he was scolded for ‘making things happen’, he didn’t make any attempt to suppress his true nature, nor did he ever imagine that he had the power to do so.
_{J.K. Rowling's new website - FAQ}

The implication seems to be that not knowing about magic isn't enough, you have to be consciously repressing your magic, forcing it back inside until it finally erupts.

‘It’s a manifestation of Dark magic,’ explains Yates. ‘It’s really a
wonderful idea that Jo came up with. When a young child is prohibited
from developing their magic in a healthy, organic way, then this dark
energy can develop, and the dark energy can suddenly get out of
control and wreak havoc.’

_{^{Inside the Magic: The Making of Fantastic Beasts}}

Harry, by comparison isn't especially trying not to do magic. Rather than repressing it, it just occasionally bubbles out of him

“So Harry had been brought up by his dead mother’s sister and her husband. He had spent ten years with the Dursleys, never understanding why he kept making odd things happen without meaning to, believing the Dursleys’ story that he had got his scar in the car crash which had killed his parents.

...

And then, exactly a year ago, Hogwarts had written to Harry, and the whole story had come out.”

_{^{Harry Potter and the Chamber of Secrets, Chapter 1 (The Worst Birthday)}}

In his later year, had he remained with a vehemently anti-magical family like the Dursleys (rather than going to Hogwarts), there's a distinct possibility that he might have gone on to become an Obscurial, although that particular affliction only seems to affect younger children.

I believe the reason that the reason why a young wizard or witch developing an Obscurus is rare is because they actually have to be extremely powerful to contain the magic inside of them, as shown when Grimwald told Credence that the person he was searching for was "Exceptionally powerful", suggesting that even if a young wizard or witch tries to hide their powers they won't become an Obscurus unless they actively use their own magic to repress their own magic.

Harry Potter did not develop an obscuris because he was not supressing his magical power, as he did not realise he had any. From his point of view, strange and inexplicable things just happened around him. There was no suppression going on. He was actually expressing his natural ability, without his own knowledge.

Just gonna leave my answer here from movies stack:

Harry never knew he was a Wizard or could use magic in the first place to suppress magic. He just thought it was "strange"

The Dursleys never liked to acknowledged Harry's magical roots. They just wanted to ignore it. Harry was forbidden to do or mention magic but not beaten about it the way Credence was beaten. He was hidden from guests, lied to about with neighbours, changed schools, but not suppressed.

Also most importantly, Harry never held back magic. He made the glass disappear, he ended up on his muggle school ceiling top when Dudley and his gang were chasing him, he ballooned up Aunt Marge. So he had his own outbursts but they we not exactly parasitical like Credence's

An obscurial arises from forcefully suppressing magic. Harry was never suppressed, and was in a lesser horrible condition as an obscurial (eg:Credence) was.

That is probably the reason Harry never was an obscurial.

One example of someone who might have been an Obscurial was Arianna Dumbledore.

"It destroyed her, what they did: She was never right again,"
Aberforth said. "She wouldn’t use magic, but she couldn’t get rid of
it; it turned inward and drove her mad, it exploded out of her when
she couldn’t control it, and at times she was strange and dangerous.
But mostly she was sweet and scared and harmless."

"[If] the Ministry had known what Ariana had become, she’d have been
locked up in St. Mungo’s for good. They’d have seen her as a serious
threat to the International Statute of Secrecy, unbalanced like she
was, with magic exploding out of her at moments when she couldn’t keep
it in any longer."