Because they are extremely good! They use a single drive unit that still manages to produce full treble extension and flat response to about 50hz. The single driver design eliminates all of the phase and crossover distortions inherent to most speaker designs, and gives many of the benefits that we consider perfectly normal in headphone listening.
It also has a heavily engineered cabinet that is completely non-resonant, and it looks pretty hot too!
And in the speaker world, as a flagship desgin, they really aren't 'so expensive', unfortunately!