As PN said,
Good endurance, good lookout platform, good numbers but above all, good crews.
Plus good equipment and a massive bomb bay, capable of carrying a lot of goodies. ASV 21 was one of the best ASW radars out there (IMHO much better than the P3C's we worked with) and even the 1C sonics were good for their time, but the airframe was getting old, with little room for more modern equipment modifications. The Mk 3, although newer, was heavier and running out of fatigue hours, and even the Mk 2 wasn't far behind, so the decision was forced through the Treasury to replace it. Then came the AEW requirement and it was decided to fit the AN/APS 20 from the Gannet (and previously the Avenger!) in the 'youngest' airframes - and even they needed new mainspars after a few years service. The AN/APS 20 seemed a real backward step from the ASV21 - it worked but was always being tinkered with, and was much earlier technology.
(Of note I seem to remember an ASWDU aircraft was fitted with a MAD boom for a trial, but it wasn't very successful and later removed)