I think there are two reasons.
One is that people here are more interested in the basic quality of the keyboard. That is why a brand like Cooler master, with their Costar made keyboards, get a lot of attention and get the nod for some of the most "bang-for-buck" mechanical keyboards out there (despite being a gaming brand).
The other one is that most of those brands make keyboards that are made to appeal aesthetically to gamers. There are plenty of gamers on geekhack, but when it comes to keyboards, they are keyboards enthusiasts first and foremost. The bright lights, glossy plastic, pictures of ninjas and so on that you find on the gaming branded peripherals, typically do not appeal to anyone except the target audience. Again, Cooler master in the exception, as their keyboards do not look like they were inspired by a sci-fi movie, and they keep it pretty basic.
That said, most of those brands do create good products. But they are just not up to par when it comes to some other brands out there. Corsair for example makes some unique looking keyboards that are quite popular. However, with some of the issues they had (like the dying leds in their keyboards) they are not preferred here to some other more established brands.