MD8x aircrafts were the last batch of aircrafts using PW JT8D of 1950's design, with low bypass ratio, hence high fuel consumption & high noise level. Susprisingly some DC8 were changed to more advanced, high bypass CFM56 engines in 1980's.

So a question is risen: Why PW JT8D of backward design, low bypass ratio, hence high fuel consumption & high noise were used on MD8x instead of better CFM56 engines used on DC8-70?

Thanks.