User blog comment:LordLuminous/Fairy Tail Chapter 411 Theories and Predictions/@comment-121.54.54.151-20141127014316/@comment-9595671-20141201081742

Yeah, remember that a guild member only learns of Lumen Histoire if the current Master deems them worthy of being the next. Considering the power of Lumen Histoire and how Mavis cried when she learned Hades turned to the dark side the guild master need someone they feel is ready to handle the responsibility.

As most people say, with great power comes with great responsibilities, its quite likely that Makarov doesn't want them to remember because this power could be misused and Hades proved that the guildmaster's choice of successor can end up being the wrong choice.

For example: some of the members of Fairy Tail like Natsu or Gajeel probably would be responsible enough for this power as they are now.