Starcraft ripped off the Warhammer 40k universe and made some improvements on it. The zerg and protoss are more interesting than the tyranids and aeldari. Then they took the Imperium of Man and turned it to space rednecks with confederate flag bumper stickers on their ships.

At the end of the day Starcraft was always going to tell a story with a happy ending, so the world they created to tell it was always going to be less effectively horrifying than the Warhammer world. More humane, more reliant on the hope of people coming together to overcome evil

Meanwhile WH40K centers on a theocratic fascist space-feudalism where space ships take shortcuts through physical reality by jumping into hell, space knights fight demons with chainsaw swords, inquisitors snoop around fishing for evidence so they can exterminate entire planets.

It's a more childish, more gleefully wicked, a simply more evil setting. And somehow this makes for a world where there is real room for stories to be told. The corruption of Eisenhorn. The Ciaphas Cain comedic novels about a cowardly officer. The "Rites of Passage" novel.

It is, to put it simply, so much more fun a setting, so much more awful a setting, than the Starcraft imitation manages to be.

And, as Serg just pointed out: Starcraft has no orcs! Awful, just awful!