One reason for the longevity of the B-52 was its ubiquity as a bomb truck during the Vietnam war and, much later, during GW I, at whilst stage the addition of PGMs has continued to show its value to the USAF in operations where a long range/endurance is of benefit.
Additionally it’s design makes upgrades and refurbishment economic - unlike the B-1, whose centrebody and wing is, apparently, a nightmare to work on and impossible to replace.
The Vulcan structure, especially the wing, I am led to believe, would also have been a nightmare to refurbish - and the Vulcan fleet in total only ever dropped a single 1000lb bomb on a target in anger*.
(* Yes, I know, the string was dropped diagonally so only 1 or 2 were supposed to hit the runway - and one if the Dhrikes hit the TPS-43 generator. But a bit of hyperbole for effect...)