Many critics and grammarians have insisted that so must be followed by that in formal writing when used to introduce a clause giving the reason for or purpose of an action: He stayed so that he could see the second feature.
Both so (that) are acceptably used to introduce clauses that state a result or consequence: The Bay Bridge was still closed, so (that) the drive took an hour.
What are the reasons behind such a different treatment?
Interesting. I would disagree with this,
Both so (that) are acceptably used to introduce clauses that state a result or consequence: The Bay Bridge was still closed, so (that) the drive took an hour.
and propose that when stating a result or consequence, it should simply be "so". Not "so that".
In terms of giving the reason or purpose of an action, use "so that". As often happens in English, in many situations, "that" becomes optional. Therefore, "so (that)".
What are the reasons behind such a different treatment?
Based on the analysis above, the reason for the different treatment is you have two semantic cases. Either a result, or a directed purpose. They are each expressed in their own way. Any time you communicate separate ideas (apples or oranges), you may choose distinct wording.
