Devs said on numerous ocassions they will release the last 3 masteries with the final release of the game 1.0
Furthermore, they said they will try (not guaranteed) to release 1.0 before the end of the current year.
So, what do I understand from all of this?
They release runemaster now because they will delay 1.0 again.
We realized that we couldn't do all 3 masteries in a single patch cycle and we didn't want to sit on the runemaster for a whole patch cycle so we are releasing it early. This has no negative implications for 1.0.