Encontrei isso na seção de perguntas frequentes do site pessoal da JKR .
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.
JKR: 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.