Scans_Daily

If I were to guess, two fronts:

First of all, the game came out during the "walking Simulator" craze. That is, games boasted a lack of actual gameplay as a feature and many people defended on the basis that it was seeking to be more "artistic" or "tell a a story" than in being a game. by "telling a story" I mean go around looking for journal entries or whatever and piece together the story from there.
I wont' go into too much detail, But I will say that Stanley Parable was one of the better ones by a combination of clever use of map layou, witty dialogue from the narrator and good humor (or to put it another way, if you don't like the dialogue and the humor, the game is painfully dull)

Second, the game came at a point when the idea of "branching stories, multiple ending, Player Choice (Not in the "Microtransactions are OPTIONAL" sense.)" as game buzzwords was starting to show up and it was considered a must have (this was like one year before Telltale's The Walking Dead and when Bioware's name wasn't a laughingstock).
So at the time when the game was released, everyone loved it because they felt it was a great example of a game with multiple endings and trying to find all of them.
Of course, Stanley Parable was partially a commentary/parody of this idea of the player "really" having a choice, since the common theme within the game is that all of your choices were actually programmed by someone and even the narrator is nothing more than following a script.
This is more clearly dictated with the female narrator.