It's just a function of when the aircraft was designed. The WC-135 aircraft are modified C-135s produced in the 60's. The C-135 was designed in the mid-50's. At

It's just a function of when the aircraft was designed. The WC-135 aircraft are modified C-135s produced in the 60's. The C-135 was designed in the mid-50's. At the time, high-bypass engines simply didn't exist yet.