Koenma: Sacred Energy is the highest echelon of power that any being can harness - but how COULD you? It takes forty years of discipline, at least.
Shinobu Sensui: Forty years shared by seven personalities is only six. Of course, even the training's no guarantee. You need to be receptive to it, having the inherent desire to correct the world around you - something Genkai discovered she did NOT possess.