Barbarian spawn code (base CivIV:BtS) takes a lot of cues from the average number of cities that the players (human and AI) in the game have settled. For example, the cue that causes barabarians to start invading your borders instead of wandering around outside is triggered by the average number of cities each player has passing a certain threshold.
It is possible the barbarians are not allowed to have cities on unsettled continents. Or possibly the barbarians could settle their cities there, but there is a cap on the number of cities they can settle and they prefer cities on continents with other players, and therefore haven't yet chosen to settle on other continents.