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There was not a single reason for this, and the reasons would not even be consistent from one battle to the next. I'm more of a generalist on the Pacific War and not an expert on one area, so hopefully some other voices can provide more detail here.
There is at least one common thread through many of these battles: The Japanese had no avenue to withdraw or retreat from the island, and surrendered only rarely (and then in small numbers). That meant there were many battles in which almost every Japanese casualty was KIA. If you combine the total American casualties (WIA and KIA) and compare to the Japanese casualties, the numbers are still lopsided but make much more sense.
Here are some examples. This list is not meant to be exhaustive, but includes several battles that most people would probably think of first. It also includes several actions that are worth exploring for understanding how tactics evolved during the war.
| Battle | Date | Japanese KIA | Japanese WIA/POW | Total Japanese casualties | American KIA | American WIA | Total American casualties |
|---|---|---|---|---|---|---|---|
| Tarawa* | Nov. 1943 | 4,690 | 17 | 4,707 | 1,009 | 2,101 | 3,110 |
| Saipan | June/July 1944 | ~23,000 | 1,780& | ~24,780 | 2,949 | 10,464 | 13,413 |
| Peleliu | Sept.-Nov. 1944 | "Nearly 11,000" | 301 | ~11,000 | 1,544 | 6,843 | 8,387 |
| Iwo Jima+ | Feb./March 1945 | ~19,000 | 1,083 | ~20,000 | 6,871 | 19,217 | 26,088 |
| Okinawa | April-June 1945 | ~110,000 | ~10,000 | ~120,000 | 12,520 | 36,631 | 49,151 |
Notes
*I'm omitting the US Navy casualties from the loss of the USS Liscome Bay. We can also presume that among the 4,690 Japanese dead were a number of laborers; I omitted the surrendered laborers included in the article as I would not consider laborers, often forced from countries and areas other than Japan, to be military combatants. However, a number of Korean laborers were killed, and it isn't possible to separate them out from the other KIAs, so that figure includes them.
&This figure again seems to include mostly Korean laborers.
+The caves on Iwo Jima made it difficult to account accurately for Japanese casualties. This source states that about 3,000 Japanese remained in hiding on the island, but I'm using the 1,083 figure.
So casualties were not always lopsided, though KIAs were. This is unsurprising, given that the Japanese forces in these battles had no option to withdraw from the area, little opportunity to retreat (though there were some tactical retreats), and were averse to surrender.
Let's discuss a few common factors in many of the battles in the theater.
Pre-invasion bombardment
Most American amphibious invasions in the Pacific were prefaced by massive bombardment campaigns both from naval gunnery and aircraft (mostly carrier-based planes). The first contested amphibious invasion of the Pacific war was at Tarawa Atoll in November 1943 (the primary Japanese defenses were on Betio Island). The Japanese had prepared the island for invasion and intended to meet the Americans on the beaches of the small island. The Americans arrived with good intelligence and reconnaissance of the island, and intended to pound the island defenses before putting men ashore.
American carrier-based aircraft began bombing the island on November 18, followed by bombardment from a handful of cruisers and destroyers on November 19.^1 On the morning of the invasion, November 20, the aircraft carriers launched another round of early morning strikes that were done shortly after 0600. Then the fleet opened up. To the best of my knowledge, US Marines experienced one bombardment from a pair of Japanese battleships once at Guadalcanal. They described it as a uniquely terrifying experience, different than anything else they encountered in combat. The Japanese on Betio endured almost two hours of naval gunnery attacks led by the battleships USS Maryland and USS Tennessee, then the carrier planes made a final run at the beaches shortly before the first wave arrived. The ships used that small break to begin shifting their fire inland. Of course, Betio was only about 1,000 feet across in many places. It's not a big island. The Marines who were watching the bombardment from their landing craft or from the fleet didn't expect that anything could still be alive on such a small island after such an intense bombardment.
They were wrong. The first waves struggled to reach the beaches because of an unusual tide; the first waves that did reach the beaches were immediately pinned down. The Japanese sank a handful of the LVTs (landing vehicle, tracked) carrying Marines and most of the tanks used in the attack were sunk or knocked out quickly. The Marines eventually overcame the defenses on the beach and captured the island by November 23, but it was determined afterward that the pre-invasion bombardment was totally insufficient. Still, it's clear the bombardments would have killed some Japanese and assisted in the invasion. In particular, Japanese gun emplacements that chose to fire on American warships drew heavy fire and were often knocked out. The Japanese learned that they were usually better off not firing at warships after some of these experiences in early invasions.
By late in the war, pre-invasion bombardments were much heavier. Iwo Jima was bombarded by the fleet for three days; the Marines had requested 10 days of bombardment. The island was only eight square miles. The American fleet that bombarded Iwo Jima also would have been much larger than the one that hit Betio; there were 6 battleships on the first day of the bombardment at Iwo Jima, then 3 more added on the next day.^2 There were also more than 20 aircraft carriers of varying sizes supporting the landing and dozens of cruisers and destroyers. The Americans always understood that heavy naval bombardments were necessary in advance of amphibious invasions. They just learned during the war that they needed even more firepower than they had expected originally.
Starvation and disease
Thousands of Japanese soldiers died in combat areas due to malnutrition, disease, or a combination of the two. The Japanese had constant struggles to supply their far-reaching island positions as early as late 1942, when the Guadalcanal campaign began, and it only got worse as the war continued American submarines sank ships in ever-greater numbers. Guadalcanal is an easy example, because even the Americans struggled to deliver enough food and medicine to the island. Malaria was pervasive. It was even worse for the Japanese. In his book "The Conquering Tide," Ian W. Toll reports that in December 1942 the Japanese on Guadalcanal were losing 40-50 soldiers each day due to fighting, but close to triple that each day to disease and starvation. The situation managed to deteriorate even further before the Japanese evacuated Guadalcanal in February 1943. From Toll:
The bodies of some 16,800 Japanese were left behind on Guadalcanal, many unburied. Those who were rescued were little more than scarecrows. Their knees and elbows bulged out from their shrunken limbs. Their hair and fingernails had stopped growing. Their buttocks had wasted away to the extent that their anuses were exposed, and they suffered uncontrollable diarrhea.
Toll estimates that 9,000 of 14,700 Japanese dead and missing on Guadalcanal came from disease and starvation.^3
In addition, the Allies chose to bypass several well-defended Japanese positions. The Americans would generally sweep the island of its air forces before moving on. Rabaul was a heavily defended Japanese base that the Allies never invaded; the soldiers there were left for the remainder of the war. Thousands of Japanese soldiers would have died not in the battles we're discussing here, but without ever seeing combat against the Allies.
The American logistical effort in World War II was legendary, while the Japanese struggled throughout the conflict. It was an issue even for basics for the entire war; some troops living in caves on Iwo Jima became combat ineffective because they had exhausted their supply of water (stored in jugs) while they would have otherwise been able to continue fighting.
This post by /u/wotan_weevil may answer your question:
To expand on /u/wotan_weevil's linked answer, another important aspect is that the Japanese army simply was not equipped with heavy weapons to the same degree as the Americans, and they were well aware of this difference. I talk in this post how the Japanese army favored a doctrine of "spiritual superiority", relying on Japanese soldiers and their supposedly "unique" Japanese values of courage, self-sacrifice, and determination in order to keep up offensive pressure on their enemy. This was driven heavily by the acceptance of senior officers in the IJA that Japan, with its limited industrial base, would never be able to match their potential enemies in terms of material superiority. To make up for this deficiency, they would have to rely on intense spiritual training to ensure Japanese soldiers would have a greater will to victory, and this would carry them to victory in spite of the material superiority of their enemy. Even in situations unlike the islands of the Pacific, like the War in China or the border war with the Soviet Union, soldiers of the IJA had a tendency for favoring self-sacrifice as a solution to tactical problems, and soldiers who did so were lauded in the press as heroes, regardless of whether their efforts succeeded or not. As such, in many ways higher Japanese casualties were almost to be expected with the expectation that the willingness of these soldiers to throw away their lives as "human bullets" would secure victory for Japan. From the perspective of the IJA, such casualty figures were something to be praised as showing the devotion, courage, and dedication expected of an IJA soldier, rather than something to be avoided, and indeed there are many cases where a unit's failure to go to its death was presented as a clear sign of that unit lacking those qualities.