Porque Voldemort ordenou que eles o deixassem.
‘No!’ roared Snape’s voice and the pain stopped as suddenly as it had started; Harry lay curled on the dark grass, clutching his wand and panting; somewhere above him Snape was shouting, ‘Have you forgotten our orders? Potter belongs to the Dark Lord – we are to leave him! Go! Go!’
O que não está claro é por que ele ordenou que eles o deixassem. Eu pessoalmente errei do lado de Snape intencionalmente interpretando mal as ordens de Voldemort de que Harry Potter deveria ser deixado no local.