The B-52 is designed to fly high-altitude at high subsonic speed. And they’re not very maneuverable. They’re meant to fly straight to a target, drop a payload, and GTFO.
Gunships fly low and slow, banking in a circle to keep their guns on target. For a B-52 to do the same maneuver, it’d have to be higher and much further away.
This is like saying we should make a Chinook bomber because it could carry a lot of bombs. Like yeah, it could, but there’s better options.