Technical Hombres: Why are high end speakers generally of lower impedance?

I wasn't aware that they are. There's no benefit to low impedance except in advertising copy, where sensitivity quoted 1m/2.83v will be inflated for a lower than 8 ohm speaker. An amp technically will deliver more power into a lower impedance load, but as speaker output is limited by the driver displacement there's very seldom any audible difference.

Impedance is one of a myriad of design trades. Like Bill I am not sure the majority of high-end speakers are lower than 8-ohm nominal but I have not looked. Some designs, electrostats, planer-dynamics and ribbons included, tend to exhibit lower impedance because they work under different principles and/or have less wire than a typical voice coil. Others choose to use less wire or put woofers (etc.) in parallel, and so forth. Also note dynamic impedance is rarely the same as the dc wire resistance. I am not aware of any strong arguments for or against lower-impedance speakers from a speaker design standpoint, but it is not my field.