I don't know where you got that, but Ford and GM both have different ways to go around getting the job done. For me personally, the Mustang "Terminator" is the one to buy since it was the first in the Mustang lineup with IRS and a DOHC 32 Valve V8. Unfortunately, economics and public demands meant that they had to drop the IRS in favour of a live-axle rear.
Even now, if I'm not mistaken (correct me if I'm wrong) it's the only indigenous mid-priced RWD V8 sports car from a major US manufacturer. The Camaro is a Holden, and the Charger / Challenger are old E-classes from the DaimlerChrysler days. Yet the Mustang beats them all even though in theory, the live axle rear is supposed to make it lazy and uncontrolled in turns. I'm not counting the Corvette since it's more of a competitor against the 911 than the Mustang due to its price.
I'm not sure how you say that it's not being sold as much as its competitors?