My guess (based purely on speculation) is that they're doing this to tighten the society's control on what is said in the public talks, i.e., to lessen the amount of "personal opinions" that the public speaker can introduce into the talk.
As an aside, this will probably come in handy in those kingdom halls where several congregations meet on Sunday. It should give the people more time to get out and let the other congregation come in. I knew of some kingdom halls that had to schedule meetings on Saturdays because they had too many congregations meeting there.