Well, it is not so much the case that this was a specific or unique case of the clergy becoming more powerful. All throughout Europe, the 11th and 12th centuries saw a burst of religiosity which created a more pious group of Churchmen. Furthermore, the church had lot of property bequeathed to it. However, to be clear, the church was one of the most powerful, if not the most powerful, institutions in society long before this period, and would remain so long after it. For instance, Cardinal Richelieu came out of the church, prince-bishops were important players in the Holy Roman Empire until its demise, or in earlier periods the Church was a central institution under the Carolingians and their Merovingian predecessors.