Outside of a few of the early battles, there was no order to retreat, and eventually, it became impossible to evacuate forces. That led to 100% casualties at times. The often futile banzai charges also stumped attempts to prolong battles and cause more US casualties the way the Japanese dud in later battles.
Also, American ground forces for much of the war were just qualitatively better, with better weapons, tactics, and training adaptable for modern warfare. The US went into battle with better tanks, artillery, mortars, machine guns, and infantry weapons. This was not the case in Europe all of the time, but it was so in the Pacific after Guadalcanal. Japanese forces overtime degraded in quality, being more and more the bottom of the barrel, and by the time of Okinawa, a sizable amount of the forces present were local Okinawan conscripts or Korean labourers. American forces on the other hand generally got better and better in quality due to better developed tactics (flamethrower squads, grenade cave teams, more machine guns per platoon) and better weaponry. This led to lopsided casualty counts in engagements, and the counts got even more lopsided when you combined the fact that Japanese troops were frequently cut off and malnourished without air or naval support.