Champion mastery for One For All but not for ARAM / 3v3?

Thank Riot's wonderful prioritization. They wanted to rush out the Rotating Game Mode queue, and as a result, they're busy with work on RGM modes for the foreseeable future.

If they stop work on RGM modes to enable Mastery for TT/ARAM, players will complain about the lack of new RGM modes. As such, Riot's probably going to continue working on RGM modes until there's a reasonably large catalog to cycle through. Champ Mastery's been out for a year now, yet Treeline and ARAM likely still won't get it until a few months from now at the earliest.