As you can see for yourself by searching for Darkspore on Steam, Maxis' action-RPG has been removed from Valve's digital delivery store without an official explanation. Reports seem to link this to some widespread issues with the title, but considering one of them has appeared for the first time a year ago, and the other has apparently been fixed, I'm a bit skeptical that's the reason.
Either way, thanks to Eurogamer for bringing this to our attention.