Riot should add the champion mastery levels to twisted tree lines/ dominion, because right now it feels kind of left behind, if we want the icon on our favorite champions we have to play summoners rift while some people enjoy more the 3v3 or dominion gameplay and they dont get to gain mastery level like the classic gamemode.
I saw it mentioned in an ARAM thread that the system would need to be tuned around that mode in order for it to be placed on there, so pretty sure the same can be said about the other maps. Each of them do have slightly different game play overall, so the system would need to look at different aspects in order for it to correctly assign the points... so that might take them a bit more time.
