OG的解释是这样说的: in that is a conjunction that means in as much as; because in that has largely gone out of use, it is considered stilted and overly formal. For, meaning because, is the most appropriate choice for joining the two independent clauses of the compound sentence.
但我觉得奇怪的是,in that为什么会out of use了呢?托福作文里都还经常出现的呀,这种理由不太充分啊。