Por que esse personagem não era um Obscurial?

Nenhum deles estava suprimindo seu poder mágico

Newt é bastante claro sobre isso no roteiro; um Obscurus só se desenvolve quando a magia é intencionalmente suprimida:

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)}

Os primeiros capítulos da Pedra Filosofal nos dão algumas dicas sobre a infância de Harry pré-Hogwarts, e é revelado que ele teve incidentes de magia acidental mesmo antes de completar onze anos:

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"}

Embora não saibamos muito sobre a infância de Hermione, presumivelmente ela teve experiências semelhantes (embora sem o flagrante abuso infantil, devo pensar).

Curiosamente, Harry parece ser o candidato perfeito para se tornar um Obscuro; desde que ele foi literalmente abusado por suas explosões mágicas, parece lógico que ele teria tentado para suprimi-los. Embora eu não tenha nenhuma evidência disso, suspeito que sua ignorância mágica é, na verdade, o que o protegeu; já que ele não tinha noção do que estava causando suas explosões, ele não sabia como reprimi-los.

Mas, independentemente de como, é claro que ele espetacularmente falhou em suprimir seus poderes mágicos; portanto, nenhum Obscuro.

Esta pergunta é respondida por Rowling no FAQ de seu novo site.

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}

A implicação parece ser que não saber sobre magia não é suficiente, você tem que estar conscientemente reprimindo sua mágica, forçando-a de volta para dentro até que finalmente exploda.

‘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, em comparação, não está especialmente tentando não fazer mágica . Em vez de reprimi-lo, ele ocasionalmente borbulha fora dele

“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)}}

Harry Potter não desenvolveu um obscuris porque ele não estava suprimindo seu poder mágico, como ele não percebeu que ele tinha algum. Do seu ponto de vista, coisas estranhas e inexplicáveis aconteciam ao seu redor. Não houve supressão acontecendo. Ele estava realmente expressando sua habilidade natural, sem seu próprio conhecimento.