Why strategic bombers are poor close-support tools

Were heavy bombers used as close-air-support assets in the Second World War and Vietnam?

Usually no: Second World War attacks were planned strikes on major targets, while B-52s had different endurance and payload advantages in Vietnam.

Operation Cobra-style strikes and attacks on rail junctions, fortifications, or troop concentrations were planned attacks on large, identifiable targets. They were operational support to a campaign rather than close air support delivered directly to troops in contact. B-52s in Vietnam could fill a somewhat different tactical role because their endurance and bomb load allowed them to wait, strike, and return. Strategic bombers remained a blunt tool when troops needed a more responsive tactical aircraft.