是不是所有的介词都可以跟宾语从句呀？

不是啊，，because of 是一个介词短语，后面不能接从句，，只能接名词，所以
His success was because of that he had been working hard这个句子不成立
其实你理解错误了，，His success was the fact that he had been working hard 在这个句子中because of 后面接的是fact这个名词而不是直接加从句，fact 后面可以接宾语从句
你也可以说His success was because of his hard-working

你举的这个例子是不对的，不能这样换．His success was because of the fact that he had been working hard 这句话中的that从句是fact的同位语．而第二句话你直接在because of后加that从句是不对的．because of后应该跟短语，不能跟句子．