Championmastery on twisted treeline

Just a simple question, Why does one not get champion mastery for playing on the twisted treeline unless its hexakill? As someone who always enjoyed playing 3v3 from time to time with friends (usually ranked) i don't get why this doesn't give mastery points, while nowadays even aram and those "fun modes" give mastery points. I can't really think of an answer to that so please enlighten me rito TY*[zombie-brand-mindblown]*

I'm not sure if this is the right board for this so bare with me.