Because people would (theoretically) have to crank up the sub's crossover so as not to "lose" that 20-40 Hz.

Assuming, of course, that they didn't have a Harman/Kardon which allows separate crossovers for all the channels...

Besides, it is debatable that below 100 Hz is where bass loses directionality. I prefer 60 Hz myself, but I have no problem living with 80. I doubt that the rolloff is so steep that I'd miss the 5 Hz.

I am the Doctor, and THIS... is my SPOON!