that 是关系代词,why 是关系副词,也就是说,that引导的定语从句中从句部分是缺成分的
如It is the reason that he explained to me.
explained后面缺宾语,that可以代替reason放在explained后面
而why引导的定语从句后面的句子是完整的
如It is the reason why he came late.
he came late.是个完整的句子