The real reason? The same time they discontinued bookstudies, they also removed the ability to do research and create a talk based solely upon the bible reading, the #2 talk, and reserved the number 3 or 4 for MS and elders.
My conclusion is that bookstudies allowed too much thought and discussion. They took that away so it's brief in KH comments, the #2 talk for just reading the bible so any comments and ideas on the reading are directly from the GB in the guide given to elders and MS, etc.
Same reason public talks area also prescripted.
Don't think, just listen, obey, and be blessed.