I believe for the more hardline stuff, they don't want it in print. They don't want to be quoted on what they really think via the pages of the publications.
If it's in a talk (which they don't want recorded, which they don't want the notes shared), the message can do its work, but years down the line, they might be left with only a vague impression of what was said. They can also discount it as one person's extreme viewpoint, not the official teaching, not the "food at the proper time" that can be found via the Watchtower. It's kind of a fudge factor so the Watchtower Society can say, "We never said that."
Often, there seems to be a "good cop/bad cop" phenomena going on.