
Originally Posted by
nedius
You're kind of thinking about this the wrong way.
It's not the size of a faction that makes them codex worthy. If that were the case, Orks and IG would probably be the only two codexes.
It's more their level of interest / importance to the story line.
40k is a very storied game or universe. Most stories focus not on the average soldier having an average day, but on the hero, the unique and the unusal. As such, a faction's inclusion in 40k is based not on their numbers or power individually, but on their 'narrative potential' (or the amount of Narrativium in their bloodstream - for the discworld/star wars fans out there!).
So, SM chapters get included as, despite their small size, a massive planet-wide battle's turning point may be a single decisive action from said chapter. The rest of the war - the mopping up and so on, is carried out by the PDF, and as such is not really that interesting on a glactic scale. So no Codex PDF.
Eldar, for all their dwindling numbers, are an important part of the galactic history, and soaked in narrative potential - a dying race, manipulative and desperate.
The Tau, the new race, accelerating into a galactic empire at speeds never before seen, again, a lot of narrative potential there.
Oddly, looking at it that way, then you start to question orks and IG. Ok, there is some narrative potential of the basic human against the horrors of the galaxy, I can see that. But then, orks have less in the way of narrative potential than just being 'stock bady guys'. Who, in their own orky way, are probably the closest thing to good guys in the galaxy... Which, I guess, is enough narrative potential on it;s own.