Just a note: They did have a Mustang with an IRS - the Cobras from the previous body style (late 90's / early 00's) had independant rears. So they have engineered that into the design of the car.
Apparently, there is still a market for live rear axle cars. Don't forget, we look at the world through rose-colored Porsche glasses. Not everyone wants to drive a car that can actually handle some twisties thrown at it!
Oh - and that would also explain the heft of the Challenger - it isn't meant to turn - it is mean to be pointed in a general direction, and get there quickly.
-Z-man.