Compression ratio is the main reason. With today's fuel and cast iron heads about 9 or 9.5 to 1 is as high as you can run with pump premium!!! Low compression strokers cost a bit more to build, but with poor quality fuel you have to build for torque and not horsepower. With the right cam and good heads, the old FE's can still put hp and torque in the 500 range and run on pump fuel!!! Not bad for an engine design that's almost 50 years old!!!!