老师说下面句子中的for the simple reason that不能换成for the simple reason why：
She didn’t answer for the simple reason that she couldn’t think of anything to say.
但记得在一本语法书上看到过：在reason 后可以用why引导定语从句，也可用that引导定语从句，也可省略不用。请问：这句为什么只用 for the simple reason that 而不能用 for the simple reason why 呢？谢谢老师！