Fantastic Beasts and Where To Find Them spoilers ahead!
With her return to the wizarding world, J.K. Rowling introduced new characters and a few new magical terms to the Harry Potter universe, one of which is the Obscurus; a magical being produced by a young witch or wizard who can't control their powers. One of the big reveals of the film was that Credence, a repressed young wizard, was an Obscurial. Apparently, this has led fans to ask J.K. Rowling why Harry Potter himself wasn't one. After all, he was untrained and suffered abuse from an unloving family up until he got his Hogwarts letter. But according to what Rowling recently published, there are crucial differences in Harry's situation and Credence's, which explain why Harry never produced 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.
Rowling goes on to explain on her website that, although the Dursleys attempted to keep Harry in the dark about his abilities in the hope that they'd just evaporate, Harry didn't grow up ashamed or afraid of his magic. In fact, he never really knew it was there...
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.
So it seems like shame and fear of magic are the primary catalysts to a child producing an Obscurus, and since Harry wasn't raised to feel ashamed of what he was, he never became an Obscurial. This would presumably also apply to Tom Riddle, who has his own history of neglect and lack of love to link him to Harry Potter and Credence, but to our knowledge, was not raised to feel ashamed or afraid of his abilities.
The topic of Obscurials has been an ongoing one since Fantastic Beasts and Where To Find Them hit theaters. Credence's fate is undetermined at the end of the movie. Though he's presumed to have been destroyed by the aurors, we know we haven't seen the last of Ezra Miller or his character in these movies. Moreover, we were told that there was originally going to be a shot of Credence boarding a ship (possibly with Newt) at the end of Fantastic Beasts. That would have confirmed that he was alive. So it may be fair to assume that the topic of Obscurials will be explored further in this film franchise.
There's also the theory that Dumbledore's younger sister, Ariana Dumbledore, was an Obscurial. Dumbledore and his brother spoke of her emotional issues and "rages" in the Harry Potter books, but never go into specifics as to how those issues manifested. Though J.K. Rowling's recent update on the topic of the obscurus doesn't confirm or debunk the theory that Ariana was an Obscurial, her answer also doesn't rule Ariana out as a witch who would fit that mold. In fact, "trauma associated with the use of magic" is pretty much Ariana's exact origin story, as we know her issues stemmed from having been abused by a bunch of muggle boys who spotted her doing magic and demanded she do it again.
Of course, Rowling hasn't commented on the Ariana theory, from what we've seen. And if there is any truth to it, we imagine she's not going to give it away before its time. It sounds like there are plenty of questions Rowling has received that she can't answer because of the films...
I'm being asked all kinds of excellent questions about Fantastic Beasts that I can't answer right now, because the answers would give away too much about future plots.
Fantastic Beasts 2 is expected to arrive in theaters November 16, 2018.