Many people wanted to be apart of the Church hierarchy because the lower ranks are more far than the manorlisum system. Also people at the bottom have a chance of become more powerful.
Pope Gregory and King Henry had a fight over the most powerful. The pope excommunicated the king causing nobles and other people to think less of the king. The pope ended up winning when the king got on his knees and begged to be let back in the church. From then on everyone knew the pope was more poweful than the king.