Less tonnage dropped due to logistical issues. Despite the A-bombs, we killed more Germans via aerial bombing because occupied Europe was just across the channel. Bomb attacks on Japan were a major undertaking because of the Pacific Ocean. The A-bomb attacks on Japan took off from 1500+ miles away on Tinian. The distance between London and Hamburg is 400+ miles away, and closer for much of occupied Europe. 10% of the German population died during WWII, as compared to less than 5% of the Japanese population.
Could the change to the firebombing campaign (where incendiaries were much lighter and used airbursts) be a reason as well?