One reason it's famous is because it used to be a prison.

一般认为，reaon 后面的关系副词可以省略，所以，看作定语从句比较好。同位语 that 省略是有条件的，那就是前面的这个名词带有动词性质，本身也是动词或有同源动词。例如：

 belief 相信 doubt 怀疑 explanation 解释 fear 害怕     feeling 感觉   hope 希望

 idea 知晓   saying 格言  rumour 传闻       risk 冒险 order 命令

 promise 答应 reply 答复       report 报道    suggestion 建议 warning 警告

以上名词的同位语从句，可是省略 that。

One reason why it's famous is because it used to be a prison.

One reason for which it's famous is because it used to be a prison.

One reason that it's famous is because it used to be a prison.

One reason it's famous is because it used to be a prison.

此外，有语法书说，reason 的表语从句要用 that 引导。其实，现在使用 because 的也很多。