IIRC, when Factions came out Canthan characters couldn't do the missions before LA either. Similarly, Tyrian characters couldn't do the island missions.
ANet's response was something along the lines of they didn't think it was fair for max characters to come back to the early, low-level areas and get skill point and experience credit for doing missions that way.
People complained that they wanted to be able to get Protector titles for multiple chapters on the same character. (Titles had just been introduced, like Factions.)
In response, ANet opened access to the early level missions for foreign characters to allow for multiple Protector titles. Foreign characters could do the early missions, but wouldn't get any skill point, experience or gold rewards for doing so. It was just for the purposes of protector and cartographer titles.
Foreign characters still cannot get quests from the early areas. That includes primary quests. I think the reason foreign characters can't get primary quests to connect the missions is understandable given the history of ANet's original intentions, player complaints, and the concessions that were made. Any foreign character in those early areas is only doing it for protector and/or cartographer titles; therefore, the player should already be familiar with the storyline.
If the player isn't so familiar with the progression, there's always the
missions guides on wiki