I think the time of year has something to do with it. This is a busy time of year for games, with stuff like GTA 5 and two new consoles coming out, along with a slew of games, a new iPad, the 2DS, etc, etc, etc...
Overlap between those things and the audience for a game like this is nowhere near 100% of course, but it's also not 0%. Releasing games this time of year carrries the risk of simply being lost in the shuffle of so much stuff going on at once.
I don't have a better explanation than that for what's happened, because it doesn't seem to be any problem with the game itself. The reception to it has been good from the people who have tried it, and the press is pretty positive too.
That said, in the future I might think about scheduling expansions for October instead of new games. An AI War expansion (for example) has a built in audience and you're going to do alright on it at any time of year. A new game needs to find an audience, and when the air is being sucked out of the room by the titans of the industry, there's that much less air for every indie & small studio trying to release at the same time. (I mean even Path of Exile is releasing soon, it's just a really busy time of year.)