请教各位老师。这是我在《剑桥雅思》范文里遇到的句子。非常不理解because后的这个从句，第一，这里没有主语，只有一个had，不知道这句话应该怎样理解。第二，knowledge后面带了一个been displayed，之前很少遇到这种只用been done的情况。不知道是我语法知识欠缺还是因为句子本身出现了错误。烦请各位老师给予指导。谢谢！

句子如下：

All of this being said, I believe that learning a different language should be not only for satisfying our physical needs, like money, but also moral needs, because never before had such a big thirst for knowledge been displayed.