Most of the model-made IG regiments were based on historical archetypes; the Catachan were based on US troops during the Vietnamese War, The Tallarn were (at least originally) a Lawrence of Arabia type but they've become more middle-eastern now, the Attilan Rough Riders are based on the mongol horseriders, the Praetorian were influenced by the Light Brigade and other regiments of that era, the Mordian guard were (in my opinion) based on the US Marine no.1 uniform, the Steel Legion were based on both the British and German uniform of WW1, and the Valhallan regiments have obvious ties with the troops of Stalinist Russia.
Likewise, the same could be said for the first founding Astartes legion, now they've been expanded, you do have some racial traits applied to the various legions, such as; the dark, almost black skin of the Salamanders, the Mediterranean lineage of the Ultramarines and Emperor's Children (who have been described as having olive skin and long noses), the mongol inspiration of the White Scars, the Scandinavian look of the Space Wolves, the decidedly Persian heritage of the Word Bearers and the Egyptian visuals of the Thousand Sons.
This trend of 'passive racist' from GW can be possibly explained by three reasons;
One: GW has to appeal to their main customer demographic, which is (let's face it), middle class, socially awkward, white caucasian male teens. Now although this doesn't have a significant bearing on this subject, it may shed some light as to why the human race in 40k has an obvious lack of other racial representation (much in the same way as statistically, female 40k gamers tend to play Tyranids).
Two: GW would rather shy away from other racial representation in their models, due to the s**tstorm of litigation that can often occur (which is the major reason why Araby has never been expanded upon beyond Warmaster). Unfortunately, unless you're a superlative sculptor, you run the risk of any 'ethnic' miniature looking almost caricatured or stereotypical (such as Al Muktar's desert dogs with their big beards and moustaches and turbans), and that such imagery is more often due to sincerely held religious observance rather than a genetic trait.
Three: as has already been pointed out, ethnic boundaries and racial groups have almost no bearing in the 40k universe, we have moved on beyond inter-species conflict and hatred mostly because we have bigger concerns with other alien races.
Indeed, territorial claims such as; "you're Pakistani" or "you're British" are just ludicrous concepts now, one might as well say "you're Tallarn" or "you're Cadian" which is no more derogatory than a statement of fact.
Likewise religion (which is arguably the cause for most of our inter-species conflict and hatred today) is all controlled by a single governing body, the Ecclesiarchy. And whilst it is true that there are many difference variances in the Imperial Church that stem from world to world, and on occasion this does lead to a war of faith, ultimately it will be over the fundamentals of doctrine rather than racial bigotry.
