My gut guess is because only a few bothered to try and with the AKG K1000 mostly and with a not too powerful speaker amps. With the arrival of HiFiMan's HE-6 all that changed. Even Fang used an old Threshold T400 to demo his HE-6. The price of the new higher end HP amps is going up, some here have discovered that vintage receivers sound good too. More options to power your HP which is good.
makes sense, good points.