Several reasons: it ALL has to do with dominance in NASCAR. The horsepower race had reached a point to where massive amounts of extra power would only return tiny speed improvements. Aerodynamics had reached a critical point.
The first aero-car was the "Charger 500", basically a stock-bodied Charger with a flush-mounted Plymouth Satellite grill installed along with the REAR bumper mounted in front. That was dramatically more aerdynamic than the sexy but deep-set grill of the stock Charger.
Also, a "plug" was made to fill in the "flying buttress" sail panel area of the back window. Beautiful to look at...horrible aerodynamics. The "plug" smoothed that out.
To handicap the big manufacturers, and allow independent racers to compete effectively, NASCAR required the manufacturers to build and sell at least 500 copies to the general public of whatever factory custom-bodied car they wanted to race. The term for this is "homologation".
The "Charger 500" was named as an homage to this.
The car was succesful at NASCAR. Very much so.
Ford responded with thier "Torino Talladega", a custom-bodied Torino.
Now things were even again.
Chrysler upped the ante with the "winged warrior", the Dodge Charger "Daytona". It was the first car to top 200 mph in NASCAR.
A little trivia: the day that the first prototype Daytona reached 200 mph for the first time during testing was the same day the first US astronauts landed on the moon: July 20, 1969.
Anyway, the Daytona was cleaning up handilly in NASCAR, and Richard Petty finally convinced Plymouth to give him a version, and they did: the 1970 Plymouth "Superbird".
Ford again responded in kind: they developed the Torino King Cobra, and the twin Mercury Cyclone Spoiler II, using all the same aero-tricks Chrysler used.
Before the cars could be sanctioned for official racing however, "Big Bill" France of NASCAR decided the aero-cars were not allowed. No more Daytonas, Superbirds and the still-born King Cobra/ Spoiler II twins never saw the light of day.
By the time the 71 Mustang was born, the aerowars at NASCAR were over. Plus, the uni-body construction of the Mustang was too flimsy for NASCAR duty.