User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-24277053-20150831113321/@comment-9595671-20150831161255

I bet after Makarov was named master, Precht told him about what happened to his mom and how Mavis being in here related to it. Probably because he felt if Makarov learned later on he might blame Mavis.

Telling him now let him know it was just an accident, and that could also count towards the Family thing. Because Mavis basically made the greatest sacrifice for her family.