I still think the primary reason is what Billy said, a lack of brothers (I know of lots of MS who had their own study group and that was unheard of 15 years ago) to conduct. If you think the average JW is happy with one less meeting night, the elders I know were giddy about only having to conduct maybe once a month (less in congos with large BOEs).
While I did see things go off the beaten path occassionally, the current "generation" of JWs are not all that inclined to get too deep into the material (in my experience). I always made it a practice of finishing a little early, especially on goodie night.
As ynot stated, I think the local congregations are becoming less close and not having the separate BS groups is a big reason for that. While EVERY single JW that I know is thrilled to have the night off from meetings, many would have rather seen the TMS/MS cancelled and the book study groups continued.
I think the WTS may have underestimated the social aspect and feeling of closeness the CBS gave to the congregations. Of course, the horse is out of the barn. They would have an extremely difficult time reinstating the extra meeting night. Maybe they knew it would be demoralizing but felt like they didn't have a choice. That meeting was typically the least attended but a sentimental favorite.