Once you have control of the airspace over your opponent then stealth takes a back seat and what you want is a (relatively) low cost way to delivery large amounts of ordinance over a long distance.

A Bomb Truck.

The B-52 does that better than anything else out there.

Throw in that much of the design was built with 50's technology originally which meant when it was ripped out and modernised you had massive amounts of space to play with which made upgrading them easier as well.

What is fascinating is that the law of diminishing returns was hit so quickly after the introduction of the jet engines for sub-sonic bombers.