Spacemarines are easily identifiable as a GW poster child though so why wouldn't they push them for all there worth. Gaurd are easily identifiable as human but so are many other companies sci fi models so maybe they become a bit samey.
I still disagree that the alien races are not well represented though the web is full of people bitching about how great or OP the necron flying circus is or the Tau battlesuit cadre, or DE venom spam and I've seen some awesome armies using different books to represent other races that have only tiny mentions in the books There was a hrud/ ghoul star denizen list on here somewhere a while back.
There is more scope for imaginative armies in the alien race books as the fluff is more sketchy, How many marines in a company or a chapter at full strength ,how many aspect warriors in a aspect lodge I bet most can answer 1 of these questions the other just comes down to personal choice.
You don't even need masses of conversions just imaginative basing and a great paint scheme maybe a few trophies to make a good back story.
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