Because the axle mount Hellwig/Addco style is more universal, easier to install, and cheaper to make.
Universal
The addco ones will fit A,62-70 B, and E bodies. The frame mount design needs to be more body specific. The end link distance needs to in close relationship to the rear end width.
Easier to Install
The axle mount style requires you drill four holes in the frame to match up with the frame width. The bar clamps to the rear end with U-bolts. A opposed to the frame mount requires you to unbolt the whole rear end and install different shock mounting plates. Also, it is tougher (but mostly possible) to get in with dual exhaust and rear end in the way. Some prior installed exhausts may be routed in the way of a frame mount style.
Cheaper to make
For axle attachment, the axle mount style has a standard U-bolts and flat plates that can be used for any similar diameter axle tube rear. The frame mount has shock plates are model
and make specific, so that drives up manufacturing costs.
The Addco/Hellwig axle mount style still gets the job done in the rear. But the unspung weight is increased 10 lbs or so.
www.firmfeel.com sells frame mount styles. They'er real nice.
I have addco/hellwig styles because I've picked them up at the swap meets over the years for $20-$40; about what I pay for used 73-76 1 1/8" front sway bars. :thumbrig:
I did get a nice used set of QA1 adjustable front shocks for $80/pr :rock: