That is a very good question. I am sure Ian and/or Alan can make exact thoughts known, but lets take a few thoughts.
As with most things in our lifetime, I would guess tehcnology has had a hand in this. More efficient driver designs have made newer, faster, higher power handling drivers. Cabinet design probably has a great deal of influence as well. They are possibly easier and cheaper to make?
You mention multiple smaller bass drivers as opposed to one larger bass driver. I can guess from different manufacturers (Axiom included I believe) that the array of drivers is/can be tuned to operate more efficiently in different frequency ranges than a driver that must reproduce all frequencies below the crossover point.
Also, as a speaker really is an electric motor which produces motion back and forth. They are built of different parts that are directly related to the size of the driver. When amplifier power (assumed to be a music note, or sound wave) is applied to the driver, it moves. Applying Newtons 1st and 2nd Laws of Motion would indicate that smaller drivers are easier to control, are quicker to change motion/direction, and easier to move.
Perhaps I am over analyzing this. Let us see if Ian/Alan can provide any information.
Regards,
BBIBH
