The reason for /to sth那个对,为什么?不是有a key to the door 这种说法吗?

The reason for /to sth那个对,为什么?不是有a key to the door 这种说法吗?
书中是The reason for sth.

首先强调一下,reason后why引导的不是同位语从句,而是定语从句.

1 reason + clause

1) reason+定语从句
a. 关系副词充当定语从句的原因状语,可以用why或that或省略.
The reason I called was to ask about the plans for Saturday. - Longman
The reason why（或者that）he doesn’t come is that he is ill. ——《英汉大词典》
The reason that [why] he died was lack of medical care. ——《用法词典》
That’s one of the reasons he always gets on my nerves. ——《用法词典》
备注：关系副词可以用for which代替.
The first question is whether the reason for which Google considered entering the wholesale broadband market in the first place still holds.
b. 关系代词充当定语从句的主语或宾语,可以用that或which,与一般的定语从句用法相同.
What is the reason that he gave for this?
What was the reason which led the slaves to leave the estates in British Guiana?

2) reason+同位语从句,用that引导.
We decided against installing new heating apparatus, for the reason that it would be too costly.
——《用法词典》
She didn’t answer for the simple reason that she couldn’t think of anything to say. - Longman

2 reason + for/of/to（小品词）
His reason for going back to school is to learn new things. - nhd.heinle.com
People give many different reasons for wanting to change jobs. – Longman
They must have had a good reason to do it. – Longman
The reasons of the German defeat were many and complex. ——《用法词典》
What was the reason for（或者of）him being late? ——《英汉大词典》

备注：
reason作“理由,原因”讲时,通常作可数名词,其前常与for搭配；作“充足理由”讲时,为不可数名词,后面多跟不定式.（以下例句来自Longman）
The tower is closed for reasons of safety.
There was no reason to panic.
We have reason to believe that the goods were stolen.
Under the circumstances, we had every reason to be suspicious.