I seem to remember in a discussion years ago that even mentioning that there were hidden images and messages in WT literature would get you disfellowshipped.
It's possible that during the 80s, the GB's grip on the art department was not all that strong. It's always puzzled me that the perpetrators of that hoax weren't tracked down and purged, Ray Franz style.
It may be related to the purported struggle between the writing and service departments. Unfortunately, Jaracz won that one and weaponised the CSA cover-ups*.
*As always, the source is snippets I've read here.