There is also the possibility, however remote, that the WTBTS has been concerned that the informal Tuesday night book studies might have been the source of some independent thinking.
Even 40+ years ago, when I was briefly a Book Study Servant, there was a lot more informal discussion about the subject at hand. More questions were asked and many more personal opinions were offered. Sometimes a newer JW or a non-baptized participant would have questions about the understanding of some particular teaching or event.
It appears that it was first a Book Study group that decided to expand their Bible reading and research that led to the "grand apostacy" that culminated in Ray Franz's leaving Bethel and eventual disfellowshipping.
By bringing everyone to the Kingdom Hall, the local elders have more control over the conduct of the book studies and can limit discussion to the material in the assigned paragraphs. In other words, it would be just like the Watchtower Study, except they would use a recent book publication and it would be a shorter meeting.