Horror is built on expectation. The thought that something dark and terrible could be lurking in the shadows is much more effective than actually seeing it.
Frictional Games' infamous breakout hit, Amnesia: The Dark Descent, struggled a bit with this unseen expectations game. Amnesia is still one of the scariest games in recent memory, even five years on. But just like any video game, the developers had to give control to players people who could move at their own pace, reload as often as they wanted, and drag Amnesia's polygonal stalkers out of the shadows and into the light. The suspicion of what could be gave way to the knowledge of what was.
Soma is also built on expectation. Specifically, it defies assumptions about the kind of stories horror games usually tell. It even defies Frictional's own marketing as strictly a follow-up to The Dark Descent, though the two do have much in common. Soma is much more than another horror game, just not in the ways you might predict.