Personally, I find this fact really easy to explain: those who download the demo are people who are already interested in card/sci-fi games. They see the title, they get curious, play a bit, and finally, since the game is nice, they buy it.
The problem is that the people who like this kind of games are often not the same ones who like your usual genres. So this creates the following annoying situation: many of your usual customers are not interested in PSCD, while players who may like it DO NOT know you and consequently DO NOT try it.
Conclusion: few downloads, but almost all who try it later buy it too, exactly as you said.
On the other hand, on Steam there are lots of card games fans. So yes, I am absolutely sure that PSCD will give you a lot of satisfaction on that platform