the reason he died so quickly is that he didn't

对不起我刚才忽略了一个问题
都可以的
因为本句子是一个同位语从句
他死得这么快的原因是不停医生劝告
reason后面的he died so quickly 是修饰reason的同位语 用why that都可以
但有时候需要注意定语从句和同位语从句的区别
可以给你举个例子
他迟到的原因是他妈妈病了
The reason why he is late is that his mother is ill.同位语从句
他给我们的理由是不可信的
The reason that he gave us is unbelievable.定语从句