I'd say there can be a couple of reasons:
1) They have something they don't want publicly known or at least known by JWs.
2) They want it to appear they have uber important info. in order to maintain their level of importance and therefore control over JWs.
Secrecy is usually a tool maintained and used by leaders of a group to ensure a certain level of control over the members of that group.
I suspect an update to the Flock book will have to be made as it is now public. That was their uber secret elder handbook which is not so secret any longer (even though we all had access to it years ago too).