I think the system requirements are probably this game's biggest obstacle. Given the standard length of development cycles, SH's requirement for DX10 means that it was conceived well before Windows 7 was an item of note on the horizon, meaning it was envisioned from a very early stage as Vista-yes, XP-no. It was then released around the same time as Win7, and thereby ran into the same problems that console release titles often have: with little to no preexisting userbase to support it, sales were less than stellar.
Smart move would have been to go the Crysis route, and make all the DX10-only content modular - though whether that's even necessary is doubtful given the existence of an app that unlocks Crysis' supposed DX10-only features on Windows XP.
Bottom line is, Windows XP remained the dominant operating system despite Vista's efforts to make people love it, and Windows 7 will be dealing with Vista fallout for some time before it starts to phase into common use - if indeed it ever does. Releasing a game that can't even be played on the dominant PC operating system is a shot in the foot however you look at it.
Addendum: Statistical breakdown of the most common operating systems on the web
http://www.w3schools.com/browsers/browsers_os.asp
Assuming these statistics are accurate globally, I submit that when a game is playable by less than a third of Windows PCs at release, it's going to encounter some issues with sales.
On the other hand, it's possible I'm just talking out of my ass and bitter about the fact that I can't play Shattered Horizon.
