This seems like excess money chasing spending opportunities to me.
The US's large engagement in Afghanistan is supported via a 1000 mile truck based logistics chain maintained over less than spectacular Pakistani highways. The true cost per gallon of fuel there is surely several times the norm, yet no one complains. So I don't buy the idea that economics are a factor here either.
The minimal usage of the B-52s, even lower than that of corporate jets, make any economic return on a multi billion re-engining deeply implausible.
Meanwhile, to claim that parts for a 70 year old design cannot be efficiently made any more is just silly. Maybe they would not be made as badly as before, because metallurgy has progressed, but there is no conceivable reason that TF-33s could not be replicated today, at much lower cost than re-certifying the B-52 for a whole new propulsion system.