When RuneScape and Old School RuneScape Wikis forked, the license was changed from CC-By-SA to CC-BY-SA-NC. As a result, all new changes come under the new license. To copy changes back to Fandom, they'd need to update their license to match and as I understand it so forfeit some of their ability to use so much advertising. I'm not sure if that license update was also applied to Minecraft or LoL, but it's a possibility.

The only use of CC-BY-SA-NC on Fandom I can recall is Memory Alpha (Star Trek Wiki) which was acquired with that license. There were some extra hoops to jump through to be able to advertise there - I think they granted themselves the ability to do so through Terms of Use that override the license.