It may also have something to do with the fact that a lot of (military) aircraft radios used crystals for their radios - none of that dialing up any frequency like you do today. I have a vague memory of having to specify the (12?) frequencies I wanted for a landaway in a Chipmunk, hence the NATO common frequencies made transits and approaches a lot easier. Even in the 70's our ILS's frequencies were still crystal controlled, so if you diverted it was not uncommon to find you couldn't use the ILS when you wanted it the most!