The official reason is that it's a loving arrangement to not overburden the "brothers", and let JWs have more time for personal study. Personally I think the high dropout rate scares them, so they try to ease things up a little - there are lots of such small changes, like pioneer hours dropped, less literature, shorter assemblies, etc.
I still can't believe how they can talk out of both sides of their mouths. Just weeks before the announcement about dropping the book study, the elders were still talking about how important it is to be present at every single meeting. Two weeks later that same elder says "what a loving arrangement to have one less meeting". I was the only one in the audience who was not clapping, (which is a paradox, because I was probably the happiest to have less meetings), because the hypochrisy was just so shocking.
Not long after that I realized that if dropping one meeting is loving, then dropping all five is five times better, so that's what I did