JT8’s have been around for along time and are not very current. Why can’t they put a more current CFM’s like are on the KC-135’s?
When the process to re-engine the plane started, The JT8s were probably the newest technology. ;)
I am not sure about all of the technical considerations, but the E-8 is a heavy SOB with a lot of electronics equipment requiring a lot of engine driven generators. Maybe the JT8s met that need better?