From what I can remember, and I might misremember some tiny stuffs; the chaos god Slaanesh was already kinda forming in the warp. At some point these space elves just got so bored with their lives, as they were very well off, they started to engage in these steadily worsening acts of sex, torture, mutilation etc; they even let it be done to themselves for, you know, a change of pace.

This sudden influx of chaotic energy caused by these deviant acts on an interstellar scale gave Slaanesh such a growthspurt that it instantly got pushed to creation, creating a violent warp/psychic explosion. As the space elves essentially “created” the chaos god they all suffered from this explosion, causing instant death. Only the elves in the webway where spared.

So actually, only a handful of Eldar as a race remain. I think the Dark Eldar learned their lesson now not to over indulge?