Masteries have been out for months

They points and grade you get is being clalculated on factors (csing, KDA, warding, ...) that were adujsted to Summoner's Rift. Readjusting and keeping them fair and balanced is almost impossible for ARAM or Dominion since there are completely different objectives to achieve. For example last-hitting minions is mandatory for the lane roles, but on ARAM minions are more in the crossfire. It's probably possible to readjust/change those factors, but I think that's not Rito's main focus right now.

As someone who has played almost exclusively ARAM and FGMs since Legend of the Poro King was on the PBE i would really appreciate Mastery on non-Rift game modes even if it's a reduced amount of points. Retroactive points would be amazing.