his has always felt like a very strange thing to me.

The Eldar are a dying race, slowly dwindling away. So by definition there has to be more Eldar alive during the Horus Heresy than during the 41st Millennium. Its only 200 years since the Fall. Despite this the Craftworlds play a MAJOR role in the story of 40k. Either acting as direct allies to the Imperium against Chaos, especially Slannesh who is their Great Enemy and by manipulating events in a way that will save the Eldar. We see plenty of examples of this in 40k. Even in the Gathering Storm, a story overwhelmingly about the Return of Guilliman they play a pivotal role in those proceedings. Plus, the Eldar frequently find themselves targeted by Chaos anyway, especially Slannesh.

So to me its never made much sense how minor a role the Eldar play in the Horus Heresy. From what I've read theres a few excuses used for this:

1) The Eldar haven't recovered from the Great Crusade:

This argument makes no sense because the Eldar are incapable of recovering as a species: every Eldar lost means the subsequent generation will be smaller due to She Who Thirsts and the inability for Eldar souls to be reincarnated. A good example is Craftworld Lyanden. This is before the Tyranids wiped out this most populous craftworld which means it would still have far more people on it than the few thousand it does. Despite this, all of the Craftworlds are major players in the galaxy. So there are more Eldar during the Horus Heresy.

2) The Eldar are part of the weird Alpha Legion conspiracy so are hoping the Imperium and Chaos destroy eachother without any Eldar dying:

This is contradicted by Eldrad showing up during the events of Fulgrim. Its also extremely unlikely that every craftworld could or would agree to this insane plan. It is also an extremely risky plan. If you read Ruinstorm, Sanguinious sees visions of the future in which Chaos triumphs and consumes the galaxy with Horus Dark Empire ascendant. It is obviously a very bad idea to "do nothing" in this situation and it obviously isn't the only future that might be gleamed.

Basically its very "out of character" for the Eldar to have not attacked the Traitor Legions during the Horus Heresy. You can't argue that "Horus Traitor Legions are too powerful" since the he is nowhere near as powerful as the Chaos army unleashed during the events of the Dark Imperium. If Horus wins, they die. It makes much more sense to make common cause with the Imperium; for which there is a very long precedent. Plus it doesn't make sense to have a MAJOR player in the story be essentially uninvolved in such a major galactic event which very much involves them. If Horus wins, Slannesh will use the Emperors Children to consume every Eldar soul. Plus the Eldar know that the Rhana Dhandra or final war against Chaos will come; so they should know that this won't work. They know that they must die to destroy Slannesh so they wouldn't buy that "oh yeah Horus can destroy Chaos".


Dark Eldar:

The Dark Eldar have pretty much every reason to take advantage of the Chaos and attack the Imperium and Horus Dark Empire. Both armies are distracted by their own civil war. It would be the perfect time to reave and enslave countless human worlds. As far as I am aware this is not the case. The Dark Eldar are just not acting in character and politely letting the Imperium have a breather before attacking them; which in 40k they do on a regular occurance.

Orks:

Now this is possibly the only one for which you could legit argue there is a good excuse. We know that immediately before the events of the Horus Heresy the Emperor destroyed a vast Ork Empire on Ullanor which was led by a Prime Ork. The events of the Beast Arises imply that the Orks do not recover for 2000 years when another Prime Ork rises. In fact, as far as we know, the Orks never reach that point again after this point with Ghasghull possibly on the cusp of evolving into a Prime Ork and returning his species to that more advanced stage of Waaagh.

But, we also know Orks. They never stop fighting. They breed incredibly quickly and can recover their strength very quickly. Ullanor can't have been the only Ork Empire. In Know no Fear they mention another Ork Empire near Ultramar for example. Plus it mentioned briefly that the Orks do take advantage of the civil war to attack the Imperium in book 3. However this isn't followed up on or developed. Again, you would be forgiven for thinking the Imperium is being given a polite breather whilst it fights it civil war. Which is really out of character, the Orks would probably rampage across a divided Imperium.

Also, the Orks aren't stupid. They can, in their own way, recognise when Chaos is up to no good and tend to attack Chaos. If Horus wins, they die.


Now, you could argue that all this is simply due to the limited scope of the books; focused on the Legions. That the Xenos probably did attack the Imperium and Horus during the Heresy; its just beyond the scope of the novels. The problem is that the series has gone on for so long its a glaring omission. These races play a major role in the galaxy later on. Plus, for a series that often likes to tell us "how bad things are getting" it doesn't make sense not to mention an alien assault. So the inference is that it isn't happening or is irrelevant. It also creates a major disconnect between the Horus Heresy and the rest of the lore. You would be forgiven for thinking humans are the only species in the galaxy or that the Eldar were fully dead dead. The amount of times Imperial or Chaos just waltzes into Eldar ruins or even the webway without being shot at is remarkable. Obviously in 40k the Imperium is being attacked by Chaos AND Xenos. Chaos isn't just attacking the Imperium but is a threat to all life. The story of the Horus Heresy only makes sense if the xenos don't attack the Imperium and chaos doesn't attack the xenos. Which is kind of unlikely and a bit of a plot hole.