here I thought it was because with a stock gas tank, there isn't enough room. The spring on the coil-over makes the package a lot larger in diameter than a std shock.
That may be part of it, but how many people going that far are running a stock tank? I've used the cheap coil-over kits when I didn't have time to fix sagging leaf springs. They fit with the stock tank, but they aren't as large (I believe) as the aftermarket shock/spring parts.