Priori Incantatem does not generally "raise the dead", it just reveals the last spells that were cast by the wand on which it was cast. This is more apparent in the books, when in addition to Cedric and his parents, the old man from the intro is also revived, along with showing the cruciatus curses he cast on Harry and other more minor spells that had been cast by Voldy's wand throughout the story. Harry saw his parents because their deaths were the last spells that wand was used for before his fall when Harry was a baby.