it is a place that i will never forget.为什么不是用in which.

in which或关系副词where在从句中作地点状语,当关系代词在从句中作主语和宾语时,即使这先行词是指地点的,仍然用的是which或that这个关系代词,同样,关系代词在从句里作主语和宾语时,先行词指时间,不用when,要用which或that,先行词是reason也不用why,也用which或that,比较下列各组句子.
I will never forget the day which we spent together.
I will never forget the day when we worked together.
That is the reason that he gave me for not going to college.
That is the reason why I like the movie "The Sound of Music" best.
希望我能帮助你解疑释惑.

直白地说，你并非是身处那个地方，无须用in which or where；那个地方只是你是否遗忘的对象，所以是that

place 作 forget 的宾语，所以可用that，which。in which相当于地点状语的引导词。