Battlefield conditions were a lot different in the Pacific Islands than in Manchuria. First, an island is a much smaller battlefield, and if you were forced out your position, there weren't "other" places to run and hide. Under these circumstances, you stayed in your cave or foxhole, and defended it to the death, "knowing" that everyone else was doing the same thing. People couldn't see others running, then surrendering, and then do the same thing. Nor was it easy to transmit surrender orders from Tokyo to the Pacific islands. Some isolated soldiers on remote Pacific Islands did not surrender until the 1970s, when their bodies, weapons, and ammunition were all "spent." One soldier in the Philippines didn't surrender until 1974, when his former (!) commanding officer made the trip all the way from Japan to order him to do so (he had dismissed other messages as "propaganda."
By August 1945, everyone was weary of war, and the Japanese knew that the war was lost, even though it took time for it to become "official." In the Manchurian campaign, the Soviets killed Japanese at a ratio of 7 to 1, a relationship that would only worsen as the Japanese lost their ability to maneuver, and to resupply. That's a "slaughter" by any standard, even Japanese. These troops knew that they were losing the war, so an order to surrender was believable.
On the other hand, in 1943, the "rollback" of the Japanese was just beginning, and they had every hope that valiant last ditch stands and "mutually assured" destruction on Pacific islands would force the Americans to the peace table. Memories of the victory at Pearl Harbor were still fresh (and the news of the defeat at Midway was kept from the "rank and file"). At the battle of Tarawa for instance, the Japanese inflicted some 3,800 casualties (killed and wounded) in exchange for 2,600 killed, in line with their plan. At Iwo Jima, there were about 26,000 Americans killed and wounded versus 18,000 Japanese killed and 200 prisoners, counting wounded. At Okinawa, 75,000 Americans killed and wounded, 77,000+ Japanese killed.
The Japanese had scattered one million men on Pacific islands in the hope of inflicting 1 million American casualties and thereby forcing the Americans to the peace table. Given the results of the island battles in the previous paragraph, the Japanese felt that they were "winning," to the point where the Americans dropped the atomic bomb because they feared that the Japanese would endure, and inflict, 1-2 million casualties if Japan were invaded.
Then there was the difference of enemies in the South Pacific and Manchuria. Up to that time, the Americans had been the most racist country in the world, at least to Japanese, discriminating against them in many ways, denying them the opportunity to become citizens, and finally rounding up Japanese-Americans in concentration camps(!) Japanese-American friends of mine have told me that "native" Japanese regarded Americans as "beasts" during World War II, and were surprised that their prisoners of war weren't all killed. (This point was also made by Robert Leckie in "Delivered From Evil.") Whereas the Japanese had fought the Russians in the 1904-05 war, and again at Khalkin Gol in 1938, and didn't have the stereotypes that they had of the Americans. Russians and Japanese had had coexisted more peacefully in Manchuria than Americans and Japanese in say, Hollywood, USA