Government.
Fuel efficiency standards.
Crash standards.
Now even pedestrian safety Standards.
That's sure a big part of it. The Jaguar E-type had triple SU carburetors until 1967, when the US government required emission controls. They went to dual Strombergs to meet standards. Those were 6 cylinder motors. Jaguar went to the V-12 motor to regain the lost horsepower, going from 4.2 to 5 liter displacement, but the V-12's were still not as fast as the pre-1967 cars.
It just seems like there's a groupthink with auto designers, though. There's a few cars that stand out today: the Mustang, the Challenger, the Camaro, Corvette, Pontiac Solstice, Mercedes SLS, Toyota Scion xB (not for good reasons, and others.