Plus, I’d argue that the market is fairly well saturated now with horror games in which you can’t fight back. When Amnesia launched, that was a pretty new idea to a lot of people. Now? You can’t go ten games in a Steam queue without hitting a knockoff game.
Amnesia is also a lot more YouTube/Twitch friendly because it’s very generic in terms of the setting and there isn’t much going on with respect to the plot that needs to really be attended to. It’s a haunted mansion with monsters that chase you. Soma, in contrast is less scary I think because you’re asked to think about a bunch of “deep” sci-fi concepts along the way.
Edit: ~50k on Steam at least. http://steamspy.com/app/282140