Adding my comment here: I'm a regular Steam user and noticed this game's release. It didn't interest me (I have both Penumbra games and barely played them) and I knew nothing about the game. The price is cheap for a new game ($20) and would have bought it if I was interested.
So I'd say that the reason it's not selling well is a combination of the game being too niche (I honestly do not see much of a demand for horror games) and poor marketing.
I hate the argument of blaming it in piracy. Sure, it will get pirated and it may have been pirated X times, but that doesn't mean it would have made Price * X amount of money. As far as I can tell, most pirates are not interested in paying for the games anyway so I doubt they would have paid if that was the only option. According to Valve, a lot of piracy occurs in countries where the game has staggered release dates (or none at all), such as Russia or Germany. Perhaps that is the issue?
"If you like it, they will stop making it" - Herblock's Law