Part of the reason these types of video games are fun is because they try to represent historical settings. Just like how some people (me) enjoy reading non fiction over fiction (History is just so damn interesting) I would rather play a game that reflects this history than just being a video game. For instance, I will never get to experience the dark ages, nor will I be able to through a video game. However it would be fun to see how history played out or human beings of the said time period developed their kingdoms and settled their lands.
On the OP's note. Id have to agree that Rome 2 is more geared to a general crowed. The time period just has more appeal for drama and entertainment. Movies like 300, Alexander, Gladiator, HBO's Rome. HBO's Rome was based in a time period that Shakespeare wrote multiple plays on and simply just has a lot of good conflict and drama. (EE campaign was this exact drama played out for you).
The Dark ages do not really get popular until the 800's when the Vikings, Franks and Anglo-Saxon kingdoms start to emerge. I have been waiting for a mod to reflect this time period but have been enjoying the 500 AD age mod (Clovis). anyways, this 400 AD Bs only sells to people who may know about that time period (Or has some fanboy thing with Attila but tbh Ghenghis Khan is way more popular than Attila if they were trying to use that as a selling point.)
Shakespeare didn't write any plays based on 400 AD (that I know of) and many people have no idea who the hell Clovis is (and he even came 100 years or 400 turns after the main campaign even starts) and get mad when they see the Romans throwing lead darts and wearing chainmail like armor over being a cliche roman soldier. I think that is why they put in the viking units and the Byzantine type units in Attila in the first place.
Anyways, it is an expansion... I am sure Barbarian invasion did not sell as much as Rome 1. I expect the same. Perhaps as time goes and a really good dark age kingdoms mod comes out (atleast I think so) It will pick up more people. But for anyone who watches HBO Rome or is a fan of SPARTANS or Gladiator or even reads Shakespeare, Rome 2 is the obvious favorite. Maybe there are some King Arthur fans who want to be the roman regiment in GB but other than that Attila's time period does not attract much attention from the entertainment industry. (When was the last movie on Attila ever?)
Problem is... I can play EE Rome 2 coops vanilla (mods always crash coops for some reason) and it is good and fun. Especially an Antony-Egypt one on EE campaign. Atilla on the other hand, is still broken Vanilla IMO.. I dont know how much of eastern/central Europe the Huns leveled to the ground... but I know it sure as hell didnt leave a nuclear wasteland which stretched from Berlin to Moscow... The razing cities is absurd and there MUST be a mod to fix it. That being said, any mod that does fix it leaves the migration and nomadic tribes pretty weak and changes the entire game.