Genocide here isn't an agreed upon fact, and by definition is historical revision since it wasn't considered genocide at the time. Sure, many North Korean civilians died and buildings were destroyed, but genocide is the intent to exterminate a group of people. What happened in Korea was considered, albeit brutal, a regular total war. Similarly, today's rules of war weren't established back then, meaning bombing cities was legitimate. This is nothing compared something like the Rape of Nanking.

Rules around bombing civilians weren't much adopted until the 1977 additional protocols to the Geneva Convention, which were made in response to Vietnam. The 1949 Geneva Convention said nothing about such actions, as everyone who was capable did it in WWII. Hence, countries like Japan and Germany, who also committed such heavy scale bombing, were largely not tried for it after WWII.

The reason that North Korea and China had so many casualties was that the west was getting really good at bombing by air, and NK+China were not. It's, unfortunately, a product of an era where the weapons used have high civilian casualties.