Why time travelers haven't fixed the 40K universe.

[Jmaximum]

I'd tell you more, but it'd be spoilery. Go read Horus Rising.

I did read it, way back. Perhaps I'm forgetting something.
Although, the final battle between the Emp and Horus seems to keep changing. I believe the original description was just Horus, dead Sanguinius, and the Emp on the bridge. Then in one of the later books, Oll Persson was added to it. Then regular guard were added, then Custodes, etc.

So please, spoilery away

[Psychosplodge]

It's down to individuals POV is all. The space marines seem to pay little attention at all to what normal humans are doing unless it directly affects them. There could have been a battalion of the imperial army troops stood on that bridge and it probably wouldn't be mentioned from any space marine retelling.

[Ravingbantha]

Perhaps all the changes from each version of a codex or rulebook it the changes made from time travelers. Maybe the 'missing' Dark Eldar characters were such a pain, someone went back and manipulated their time stream, to make them more 'administrative' and less 'hands on comabtive'. The 13th Black crusade was changed, because the loss of Eldrad was too much for Ulthwe to bear.