I firmly believe that the reason had nothing to do with any concern for the rank and file. steve2, I remember the point about the rising fuel prices; I think it was in a letter. However, I believe that that just provided them with an excuse. They never seemed to worry at any other times about how much it was costing us to be JWs. Heck, the little bit of extra money I would have spent driving to the book study with higher gas prices is nothing compared to all the money I spent being a JW and never earned in the first place because of missed opportunities caused by my being a JW.
I think it was either for legal reasons as mentioned in posts above or for another reason which I will explain below or a combination of the two.
That other reason has to do with the fact that, at least as far as my experience reaches, JWs at book studies were more relaxed and informal and tended to question material and discuss it more - during the meeting and before and after. I think the org now fears that.
JWdom knows it has lost theologically/doctrinally and no longer wants JWs to have deeper Bible discussions and/or to question. The org wants to control the info; it had less control at the smaller book study groups.
The JW leadership knows that it can't defend its theology and answer questions. In fact, in the last ten years or so, I think I remember JWs' being instructed in one of the publications not to write the branch offices with questions.