[nomercy400]

The source code shows all details of one of their most popular products. If it got out you can guarantee there will be new exploits, new ways of breaking the game, which in order to keep the game 'fun', will have to patch.

Also, don't forget they re-released Starcraft with better textures recently, so having your sources out there isn't going to improve that game.

Plus, it's their stuff, they decide what to do with it. You can ask all you want about the source code of a piece of software of 1995, but I bet you won't get it from any company just because it is part of history. I don't see the source of Windows 95 lying around on the internet either.