To answer the question in the subheadline: probably because it's a game from Focus Home Interactive, hardly the best-known and most celebrated of publishers. Developer Cyanide isn't a household name either, thanks to a hit-and-miss portfolio that includes stinky dungeon sim Impire and the pleasantly counter-intuitive Of Orcs and Men. Still, solid-looking gameplay footage cures a multitude of ills, and there's a whole 15 minutes of it below.
Don't have time for the video? Allow me to sell Styx to you in a sentence: it's essentially a Splinter Cell game that happens to star a 200-year-old goblin rogue who's able to clone himself using a magical substance known as Amber. Most of the Amber derives from the enormous World-Tree, which is kept inside the equally enormous Tower of Akenash, and is defended by a motley alliance of elves, humans and other fantasy factions. Your objective as the titular Styx (not to be confused with that river from Greek mythology, the rock band or, erm, sticks) is to break into the tower, hack open the tree and treat yourself to a nice hot bath of the stuff.
In terms of the tactics they support, the moves and abilities aren't especially novel, though some of them are executed in a novel way - you can poison water supplies by throwing up, and position a highly perishable clone to act as a noise-maker, luring guards right under a precarious heavy object. Still, I'm impressed by the fluidity of the action - those third to first-person transitions, for example - and the size of the areas.
Middle-earth: Shadow of Mordor probably deserves more of your time, but this could be a respectable alternative. Any thoughts?
And here's the most recent trailer. Cyanide is also working on an Xbox One adaptation of Space Hulk, the classic Warhammer 40k boardgame.