Авторского объяснения в романе нет. Есть слова Сириуса (сцена в Хижине):
“Lily and James only made you Secret-Keeper because I suggested it,” Black hissed, so venomously that Pettigrew took a step backward. “I thought it was the perfect plan… a bluff… Voldemort would be sure to come after me, would never dream they'd use a weak, talentless thing like you…”