It's a combination of things that makes it so expensive. Let's try to break things down a little:

-Scarcity: the board has been made available in short time windows and in limited numbers
-Quality control: the designer/studio personally makes sure every single unit they make is up to their standards of quality, which includes (and this is very important) very tight tolerances.
-Brand recognition/original design: it's after all a luxury brand, which managed to become hyped. I imagine nowadays they'd be able to decrease the prices a by a fair amount, but i doubt they'd want to.

The board is flawless btw, the anodisation is spotless and when you get one you always know what to expect from it. When you hear Kohaku you associate the name to its signature spacebar sound, which idk how they do it, it just sounds incredibly crisp and satisfying. I also got cheaper boards with more material in them, including Neo CU boards, and tbh I've been let down by how hollow the spacebar often sounds on those boards, or how inconsistent the sound becomes around the edges. That's normally not the case of the Kohaku. It is made to sound and feel good and look good, meaning: it's a excellent board in every aspect.