I think the two most important reason were:
1.) to reduce the workload for the elders.
2.) to make it easier for new ones to join the organisation; people in Western countries were not even willed just to start a bible study if they heard that JWs would attend meetings on three evenings per week...
Attendance of book study was not such a huge problem; in some areas book studies had even best attendance of all meetings (much nearer than kingdom hall; easier for interested ones to visit private homes than to attend a kingdom hall). But the organisation has less and less willed, healthy and capable elders, so their workload had to be reduced.
For reducing the number of weekly meetings, they had two possibilities:
a) to end one meeting in KH, and to develop the book study groups into real field service groups by combining book study and a short service meeting in private homes. (A shorter TMS could have been added to a 30 minutes talk + 45 minutes WT-study, to train brothers to talk in KH) I think that would have been the better solution: strong small field service groups; content of service meetings more flexible according to local field service needs; easier to combine theory of service meeting and practice of field service as group.
But: they have not enough elders anymore for such a change. So they choose possibility
b) to end study groups.