Either the rules say "only these races can have these dragonmarks!", to which the DM can choose to ignore that rule... or the rules say "any race can have a dragonmark, but it's a unique event!", to which the DM can ignore that rule.
Which means it all comes down to which side the writers are on... inclusiveness or exclusiveness.
As the year and editions have progressed... they usually have come down on the side of making everything open to everyone by default and letting DMs then restrict things for the flavor of their individual game worlds, then to close many things off by default in order to create a generic flavored world that DMs have to then re-open for their own world.
Which means it all comes down to which side the writers are on... inclusiveness or exclusiveness.
As the year and editions have progressed... they usually have come down on the side of making everything open to everyone by default and letting DMs then restrict things for the flavor of their individual game worlds, then to close many things off by default in order to create a generic flavored world that DMs have to then re-open for their own world.