Why B-52 was not re-engined as a more efficient 4-engine? Why B-52 was not re-engined as a more efficient (also for maintenance) 4-engine but is still an 8-engine aircraft?

In history, almost all others models were re-engined with new engines also quite different from original, some examples could be C5 Galaxy or also B737 that in latest geneations requires also to extend undercarriage or make bottom "flat" area of engines, to accomodate new bigger ones.

So why B-52 remains the original projects despite its long history? Did development cost be affordable in this very long period?
There were several proposals to do just that, none of them ever went beyond the proposal phase.

Part of the reason is almost certainly the large stockpile of spare engines the USAF had (and maybe still has), taken in part from re-engined KC-135s.

Part of the reason no doubt is also that there have been near constant plans to retire the B-52 "soon" and a major project like this was never deemed economical.