And in the rest of the world in the mid 1960s who was doing what?
USAF - EC-121 (& USN WV) based on the Constellation airliner with radar arrays above and below the fuselage. E-3 was only at the very start of its development from 1963, with no hardware until the 1970s and service entry in 1977.
USN - with the Grumman E-1B Tracer with a fixed over fuselage dome (with radar rotating within it) which lasted into the 1970s. E-2 with a rotodome only came into service in 1964. but it was much later in the decade before it could be said to be successful due to radar problems.
USSR - Tu-126 from 1965 with a rotodome (but the contra props caused clutter)
Prior to that everything carried the radar under the fuselage, unless you want to talk about the Wellington conversions in WW2 or the USN Goodyear blimps that carried the radar array inside the balloon. Studies for the RN in the 1960s included options with rotodomes as well as pods and nose and tail scanners.
So maybe in the 1960s not quite so obvious. And since the late 1980s many systems have shied away from the rotodome altogether. For example
E-7 Wedgetail, Embraer 145 AEW&C, Saab 340 AEW&C, Saab 2000 AEW&C with a fixed long over fuselage pod with an AESA radar, a layout now copied by the Chinese in their latest AEW aircraft.
Gulfstream G550 CAEW and IAI Condor with conformal radars on the fuselage sides and nose and tail (G550 only) radomes.
One disadvantage of the rotodome is the large amount of drag it develops with its heavy supporting pylons. It also requires electronic trickery to cope with the aircraft turning (when the whole scanner tips over at an angle) whereas simple scanners in nose and tail radomes can be mechanically stabilised as was already being done with radars in fighters. As with everything in life there has to be compromises.
