_ChaosFox_: There are a number of reasons.
1. Some are convinced about the primacy of Steam and see no need to release anywhere else. A delayed GOG release is usually the result of those illusions being dashed, either because the game has utterly flopped on Steam or because the mythical
long tail has proven to be ridiculously short.
2. Others are convinced that DRM is somehow "effective". Square Enix is one of those culprits.
3. Others (mostly one-man indie development teams) are convinced that the sun shines out of GabeN's arse and refuse to put any effort into releases on anything but Steam. See Elias Viglione and the Hero Siege debacle (relates to Desura, not GOG, but still), also League of Geeks with Armello. If you're wondering how this differs from 1), it's because such people are ideologically driven, while those under 1) work according to cold, hard sales figures.
4. Some might simply have not thought to release on GOG. No malice per se, just thoughtlessness really. That's really where GOG's acquisition team comes in.
5. Valve may have acquired (timed) exclusivity rights.
6. Technical issues: certain games make use of Steamworks and either need to be ported to the Galaxy API or have the Steamworks features stripped out. It's not uncommon for games that only use achievements to fail to run without Steam, simply because the developer didn't have the foresight to incorporate the API properly.
7. A game might not have passed GOG's QA standards. That statement might elicit a giggle or two from some of the regular whingers here - especially as GOG hasn't been immune to technical issues, especially with its classic games - but it holds true that certain games on Steam have such technical problems that it would never pass GOG's human review process.
8. A game's publishers might stipulate the use of DRM for distribution (Microsoft for instance with Dust: An Elysian Tail). Once the publishing deal expires and the developer assumes control of publishing duties, they may choose to forego that approach.
Good stuff, seems to cover everything.
