I mean i understand ARAM because it's random, in Dominion and TT the same concepts apply to champion selection and skill as in SR, and there’s no reason for Champion Mastery to not apply to these maps.
It's because they said so right in the announcement. They want to make sure the feature is stable before they look into placing it in other modes.
I'd bet it's more because some champions are particularly imbalanced in the other modes. 55%+ win rates are almost unheard of on the rift, but TT/Dominion have them more frequently.
Because Bots maybe? I'm not sure really.
