Riot has sead that they want to focus on SR so they would not implement champion mastery on other maps, but this does not make sence given that there is champions mastery in other game modes.
I would guess it has a lot to do with promoting SR and the random game modes. RITO apparently doesn't want people to play ARAM and 3v3, and it is certainly doable if they wanted to. Its really stupid that I can get champion mastery for URF of all things.. as fun as it is - the only thing i master in that mode is smashing the keyboard as fast as possible.
As far as ARAM goes, I believe it's because you don't get to choose your champion. OfA technically doesn't either, but at least you have some input there. If ARAM allowed champ mastery, and you rolled up a free week champ, you'd be tempted to reroll or dodge purely because you'd get no mastery (and by extension no chests) that game. You can't even earn key fragments if you win with a free week champ. And let's face it, ARAM doesn't need any more reasons for players to dodge.
You can't just say "no more free week champs in ARAM" either, since those free champs are the only thing attempting to keep ARAM only accounts in check.
The reason why featured game mode has it, and ARAM/TT doesn't--currently--is that featured game modes tend to play out like a normal SR game. Skills in TT are different than those used on SR, and likewise, things that work in ARAM don't necessarily work in SR or even TT.
It'd be nice to be able to earn champ mastery in ARAM, since it's my preferred game mode, but in the end, I don't really care about a shiny icon on my champion portrait, or hextech chests. If or when it comes to ARAM, it'll be neat, but until then... meh, I don't really care either way. :3
