xep624: I'm quite surprised that Xenonauts is not selling fast.... It's quite a good game, I haven't played it much (backlog....), but one day I will finish it.... Brings memories from X-COM (the classic one).
Just a guess on my part, but while I think there is probably a high number of people on GOG who are quite fond of the original X-COM games, a lot if not most of them like myself actually own the X-COM games as they've been on bundles on Amazon and other sites in the last year or so at pretty ridiculously low prices. I got the entire X-COM game series including all games except for the Enemy Within expansion pack (which wasn't released yet when I bought the bundle) for $10 about 18 months ago. I sunk 70 hours into X-COM UFO defense so far and a brief trial of the newest game. I plan on going through them all in order roughly over time. Then there are the various UFO After____ games which are decent, UFO Extraterrestrials and these games have been on various promos and bundles over time also.
So I think the target market for X-COM and X-COM clone games is big but already saturated by people owning the original game series and/or clones and for many of us at least, paying $10-25 for another clone no matter how amazing it is, is still just another clone to throw on the pile. Especially since I bought the entire original XCOM official series for $10. :)
So even if it is great, it may not differentiate itself enough or bring enough to the table to cause certain gamers to crack their wallet for $10 even if they're interested. I think they'll sell more to people who don't have the originals or who liked the originals but don't want to play those old ones anymore and like the idea of a new game with a better UI and graphics etc. and think $10 is worth a try.
When the game eventually drops down to $2.99 level in promos though I bet everyone who was ever a fan of XCOM or remotely interested in it will impulse-buy in droves whether they ever play it or not. :) Right now it's at that "I gotta really really want to play it and play it right now immediately" price, but the stack of XCOM games on my shelf is in the way. :)