It is a combination of event reward limitations, grepolisscore awards, and people having been asking for it for ages. In events we typically can only offer myth units for the 5 other Gods and Goddesses because some worlds didn't have Artemis. Making it so the event on some worlds had unique non Artemis spells is added complication and work. Also Grepolisscore didn't have the Artemis related awards unlocked due to not all worlds having Artemis and now they will be unlocked with Artemis on every world. Finally, tons of people had been asking about it for quite a while.