the reason （that） he was late was that he missed the bus.前一个

the reason （that） he was late was that he missed the bus.前一个 that 引导的是什么从句呀
好多回答都说that 引导的是定语从句,但是如果引导定语从句应该在从句中做主语、表语、定语等成分呀.就像 He is the man that I saw yesterday.这里的that 做的是saw的宾语.
但是如果这里that引导的是同位语从句那that 一般不能省略呀.
那这里that引导的是什么从句呀?