As we all know, Obscurus' are created by suppressing magic - usually brought on by abuse. Considering the number of times Harry spoke of being forced to cook, living in a cupboard, never having friends, etc., it's abundantly obvious that the Dursleys emotionally, mentally, and physically abused Harry - providing him the means to become an Obscurial. It was also heavily implied that Harry truly thought any magic he did accidentally do was 'freakish', 'unnatural', and 'abnormal'. He had been conditioned to believe so by his guardians. This provides the suppression part of the situation.
Therefore, why hadn't Harry become an Obscurial?
