Acension had Mastery level gain, why cant TT and ARAM?

They said they would. No idea when they're going to do so.

It all takes time. They announced in the Rotational Queue mode that all Games in the mode would feature the Mastery Level system because they worked on it for those modes. So they have to spend the time to get it into the other maps still.

Like when we see Hexakill: Twisted Treeline it will be there as well as this weekend with Poro king. So it can work perfectly fine for those modes they know now, but they will obviously have to work around the differences needed for the calculations again.

I asked for this about a week after champion mastery was released and got the response of "We're working on it.". I know it takes a while to balance but come on its been almost a year now.