so weird!
maybe they were made for the tube heads?
My guess is that they were made for multiple cab big stage setups, but that's just a guess... I knew I wanted mine to be 4 ohm, because I knew the odds of me ever pairing it up with another cab were slim, and I could always run a pair of 4 ohm cabs off my Shuttlemax 12.0, or even slave an amp if I really wanted that much speaker power on stage... The amount of air that 215 will move is pretty amazing, though - tons of low end, but still has great punch and clarity...
Edited to add - you almost never see PH215 cabs come up on the used market...