The problem is that, when a developer adds Steam-exclusive content (trading cards, achievements, etc.), they need to create a separate executable for the non-Steam versions of the game. In most cases, a developer will work on both versions in tandem, thus allowing the patch to be released on multiple platforms simultaneously. This would, understandably, slow down development a bit.
The devs for SLAIN! clearly didn't want to waste a single day on getting this much-needed update done, so I can't really fault them for doing a staggered release in this particular instance. With the game receiving such poor reviews, they were probably anxious to get a fix out as soon as possible and, sad but true, it makes sense to cater to Steam's larger user-base if you have to pick just one version to work on.
At least we know that GOG should have the update by 8/7, so we won't have to wait much longer.