Well, I don't go to the spoiler threads anymore but aren't most of the conversations that do go on speculation due to a lack of info on the chapter? While I know there's discussions after most of the info on the chapter comes out, people have discussions in the full chapter thread and it's their choice if they don't bring the discussion into it.
Also, it can be quite confusing. Someone who doesn't wanna be spoiled to goes into the thread and sees a cam pic right in their face they'll be quite upside…there's also the truth that people use a chapter discussion thread being there as indication that the full chapter is out.
Making it one discussion would cause the person to keep checking it to see if it's still spoilers or the chapter is actually out now. There's also the fact that people want to go in their to discuss the chapter, and not have to sort through pages of people talking about the spoilers and speculating.
All in all, things are far more organized as it is.