Maighstir: http://crowdtamers.com/about/about-trevor/marketing-maestro-for-hire has been mentioned a couple times already, but a quote from it:
This is worse news than yesterday's announcement of Bohemia pulling out of gog.
skeletonbow: This is just speculation as I don't think there has been any official story, but ArmA used Gamespy for online multiplayer and Gamespy got shut down completely so there is no more online multiplayer. They updated the game apparently to add Steam support to it to continue to have online multiplayer via Steam, but GOG games can't rely on Steam for online multiplayer so their options are to have a multiplayer game on Steam and a single player only version of the same game on GOG. That is not viable because even though some players only want single player, many want multiplayer and people can easily buy a game expecting it to be multiplayer without reading the fine details and end up pissed off when they find out it is single player only, but available somewhere else multiplayer and wonder why no multiplayer here.
So that leaves a game company few options really. One is to just port the multiplayer matchmaking to Steamworks which is relatively trivial and that's what almost all game companies are going to do because of the low cost and ease of doing it, and the fact that Steam is the largest gaming platform out there with the largest reach. To keep the game on GOG.com with multiplayer would require them to either convince GOG to allow a game to use Steamworks for multiplayer matchmaking which probably is a no-go for GOG and for GOG gamers, or they would have to go and reengineer their own multiplayer matchmaking service /just/ for GOG.com which is a huge expense for an old game just for a fraction of their game players in all business reality.
So it actually makes sense from a business perspective for them to remove the game for sale from GOG.com for the time being at least. With the announcement of GOG Galaxy however today, once that service is actually launched, perhaps we will see the ArmA series get updated to support GOG Galaxy for multiplayer as well, and that would also allow gamers on GOG to play with gamers on Steam without having to have Steam accounts.