Since the character was first introduced in Star Wars: The Force Awakens, Rey has been a protagonist full of inner conflict, largely spurring from the fact that she doesn’t really know who she is and where’s she really comes from.
Without the simple explanation, the heroine must stand on her own and carve out her own path, rather than just follow one already carved out for her.
To have her learn some kind of easy answer about a noble ancestry would not only immediately zap away that important conflict, but would also totally freeze and dismantle her important journey of self-discovery.
While not every fan has reacted positively to this reveal about Rey’s parents, Rian Johnson also makes a very strong point about why he made the decision — without even mentioning the argument that giving Rey a famous relative would make the Star Wars universe feel too small.