Primarily because my rig is a little heavier than the average full size E-350. In fact its heavy enough that the original factory leaf spring suspension felt like it was welded solid to the frame, it just wasnt acceptable to me.
The system rides nice and has been virtually maintenance free since i installed it 4 years ago, other than changing the air filter on the compressor. But its far from being reasonably priced once you add the air compressor system. I also spent another $600 on a Blue Ox Tracbar kit because i didnt like the idea of those white rubbing blocks attached to the frame. Obviously theres alot more to the system than whats shown in the picture. It has a compressor system, a main air reservoir, two ping tanks, two kneeler valves, an air gauge, compressor on/off switch, kneeler switch, low air alarm, air heater, auto drain valve, two height control valves, etc....
I had to buy all the airlines and fittings seperate, it came with straight fittings and i need mostly angled fittings. Then i had build my own wiring harness with relays to run everything. I think the kit they sell now may come with a harness, not 100% sure. The basic suspension kit is $2900 then you add the compressor kit which is over $3000, then you have to buy incidentals.
This system rides better than the leaf spring setup and it automatically adjusts the ride height back to its preset height regardless how much tongue weight i put on it. Well at least until i overload it so bad the air bags explode
Never been much for following manufacturers recommendations i guess.