The group study was the least attended, and no matter how much the Org. pushed it, it remained so. I don't mean by 5% either - in some congregations the number attending was as low as 50%. Why? Well, there is not one answer to that. To some, the study books were too hard to understand (or too boring if you don't like prophesy badly interpreted). To others who rarely answer, if ever, at the KH it was very difficult to sit in a room with 15 people and stay in the background, so they just stopped coming - often when someone would remain silent the conductor would even ask them outright to read a scripture or something (this was not supposed to happen but I have seen it done many times).
In some congregations it was becoming harder to get people to offer their homes for it. One reason was the time it would take to clean and prepare the house, another that it was expensive to offer refreshments every week and another that JW children were increasingly ill behaved.
As has been mentioned, some found it too much effort to attend after a hard day's work and although everyone might notice that you were missing from a KH meeting, few would know you skipped the Group Study, so that's the one you left out.
And finally the elders...the group study conductor was also supposed to shepherd the members of his group, but this rarely happened in a positive way - it was often seen as another pressure, perhaps one pressure too much.
In the end I think the reason was simply the GB gave up on it. They have completely reversed what they said for many years that JW'S need study and association all the more so 'as the end draws near', dropped the group study, reduced the Public Talk to 30 minutes and dumbed down the study material to such an extent having a brain at all has become a hindrance.
The result? When I talk to a JW, I discover they cannot discuss the Bible at all! Once out of the safety net of what is in the paragraph they fall apart.