The reason why he didn't come was that he was ill.
The reason why ... is that...是个固定句型,表示,...的原因是...,那么是否意味着 is 前面的句子是主语从句,而that引导的句子是表语从句呢?可否这么理解。
《高考英语备考1号·速效编》
《高考英语备考1号·写作编》
《高中英语晨读晚记》
《高中英语错题笔记》
《零起点考大学英语》