Likely due to professional use cases which translate well enough to consumer needs. The sound quality from my anecdotal experience is meaningfully improved when moving from 32ohm to 80ohm, a bit from 80ohms to 250ohms and mostly inperceivably from 250ohms to 600ohms. The reason would be the difference in voice coil mass and density and at least makes intuitive sense. The unit variation is also very notable with those models and matters much more.