The combination of rarity, desirability, brand, and how interesting it is. It's one of those that seems to check every box, and therefore, is valued far higher than one would expect, or it really deserves.
People desire it because it is 'ergo training wheels', nearly everyone could use the keyboard how they like. It is also the first keyboard to use brown switches, the brown switch was literally designed specifically for this board, therefore, fans of brown switches will obviously flock to the keyboard that they were designed for.
The keyboard is a compact design, the layout is familiar, and the styling is palatable without questionable stylistic choices or horrible switch placement.
Personally, I hate brown switches and I don't prefer ergo designs(sorry TP4, I'll eat my veggies tonight). I would never buy one of these keyboards, even if they were only worth $10. It's not for everyone.