Certainly cost is a part of that. But I think it is also a matter of aesthetics. The isolation feet do not look as cool as the simple feet they have now. The lower to the floor the sub is, the nicer it looks. They do use the isolation feet on the cylinder models since the regular feet wouldn't give the down-firing cones nearly enough excursion clearance. Another thing is I don't think the isolation feet are going to really help isolate enclosure vibration when the enclosure is already so massive. At 150 lbs., the PB-4000 isn't moving anywhere, and whatever rocking motion that cabinet experiences wouldn't be damped as much by isolation as a lighter enclosure.
Besides that, for whatever good the mechanical isolation is doing, it might also have some adverse effects as well. For every action there is an equal and opposite reaction. If the motion of the driver is actually moving the cabinet, then you want that cabinet to be as immobile as possible to get the most motion out of the cone. The more you mechanically decouple the cabinet from the ground, the more the cabinet's reactive motion can negate the motion of the cone. I don't advise getting the subwoofer isolation feet for its isolation properties, I advise getting them so the subwoofer can be safely lifted by hand if needed. Peronally, I don't think mechanical isolation feet are really helping reduce vibrations that much, at least for competently constructed subwoofers. I think they might only be useful on flimsy flooring situations.
