The bombs were intended for destruction. Certainly the collateral damage was not expected--tons of nuclear researchers in the U.S. from the 40s eventually died from long term effects of the radiation. Nobody wants lingering damage from war--one of the reasons pellet-based shotguns are not used in military combat units. The purpose of combat is to kill or force surrender of your opposing, rather than intentionally cause post-war damage.

The use of shotguns was brought up after WWI at the Geneva Convention as not being particularly useful in killing, but caused a lot of long-term injuries. Thus they are no longer wielded in combat.