I was told by an elder that it was a reference book
Elders don't know, any more than the rest - he was just blagging.
The book came out in 1990. My theory was that somebody changed their minds about it as study material . Perhaps they thought it too deep for the 90's generation of dubs spoonfed the pap of dumbed down brochures and the like.. Perhaps they did not want the groups answering q's like " "explain briefly the four noble truths [of Buddhism] page 139.
Certainly we were all disappointed not to study it.