We do not allow canon OCs out of a desire to maintain a more predictable play environment for canon characters -- unlike CRAUs, canon OCs are from canon characters' "home" canon and therefore introduce headcanon that canon character players would need to deal with.
In the case of MMO OCs, additionally, many MMOs are written so that player characters occupy an important place in the story and perform plot-meaningful actions. This means that player character OCs from MMO canons would come with preexisting relationships with canon characters, which can create a confusing and unpredictable situation for canon character players (for instance: if there are five very different Warriors of Light in a game, each of whom has gone through the same situations with a canon character, who does the canon character remember going through those situations with? What if there's one Warrior of Light, the canon character player chooses to assume that character is the one their character remembers, and they drop and are replaced by a very different WoL?)
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In the case of MMO OCs, additionally, many MMOs are written so that player characters occupy an important place in the story and perform plot-meaningful actions. This means that player character OCs from MMO canons would come with preexisting relationships with canon characters, which can create a confusing and unpredictable situation for canon character players (for instance: if there are five very different Warriors of Light in a game, each of whom has gone through the same situations with a canon character, who does the canon character remember going through those situations with? What if there's one Warrior of Light, the canon character player chooses to assume that character is the one their character remembers, and they drop and are replaced by a very different WoL?)