核心问题
在张道真《实用英语语法》和章振邦等编《新编英语语法》中只指出了 the reason why 的用法：That is the reason (why) I did it. 这就是我这样做的原因。是不是只有 why 才能引导 the reason 的定语从句？
词典解答
除了 why 之外, that 也能引导 the reason 的定语从句, 而且是规定语法学派所倡导的用法。例如:
The reason (that) they didn't go to see the house is that they prefer to live in an apartment. (CGEL) 他们没去看房子的原因是他们宁愿住在公寓里。
[参见 that(adv.)²]