I think CA has a hidden reason for locking the non-roman factions.

Let's face it, the Roman factions will be the ones that got most attention and are the most pollished. The other factions will probably look slightly bland in comparison to the Romans regarding the campaignmap, because the Romans have the senate and all.

CA doesn't want to give reviewers and people who just bought the game to tell their readers and friends that their favorite faction has had less attention in the campaign, because that would be bad for sales. Reviewers won't be able to play out an entire campaign before they reach their deadline and need to write their article and will miss this flaw, and so will the public.

In other words, CA has made a cunning plan to cover up this flaw by locking the factions with less features in the campaign. Why else do you think you don't have to unlock the other factions in custom/multiplayer mode but you do with the campaign map?

Not that I really mind. I will still be happily stomping Greeks with my Bruti by the time all you guys have started their 3rd hairy barbarian campaign.