There's a reason Game's Workshop focuses much of the fiction on the Space Marines and Imperial Guard. GW has worked to create alien/non-humans that aren't just humans with funny foreheads and they feel that most writers would dilute that if they created a story from a non-human POV. I can't help but be sympathetic to that. Most of the books, movies, TV shows, and games that feature non-humans make them seem like humans from a different culture. Do you think Relic could do an Eldar or Tau PC justice? <-Non-rhetorical question
That said, the focus on the Space Marines also annoys me. It's difficult to sell me on a GRIMDARK setting when war is awesome and you get to play a technological superman. I'd love to see a game focused on a soldier in the Imperial Guard. Something by the same studio that did STALKER would be great.
IMO they should just drop the Marines are AWESUM and HEROIK, and go for the real thing: fanatics trying to defend a dying empire. Superhuman but rare, there's a limit to what even they can do.
I mean, my fav SM is Thule from Dark Crusade. Why? 'Cause he was a simple-minded idiot who could only say "I'm going to kill you alien!". He lost pretty much every battle of wits (pre-siege banter with the enemy), even against the friggin' orks.
He was just an idiot instead of an avatar of awesome.