Given their reputation for fighting to the last man in the Pacific, why did 700,000 Japanese troops surrender in Manchuria?

They aren't comparable. The Japanese soldiers in the Pacific Islands were crack troops that had been fighting bloody battles for years. They were mentally prepared to fight to the end. The Japanese army in Manchuria was just a light infantry garrison for suppressing guerillas. It hadn't fought any real armies since the Soviets and the Japanese signed the non-agression pact.

Remember the Red Army defeated even Hitler. When they attacked Manchuria the Japanese were defeated very quickly. It's basically the WW2 version of shock and awe in Iraq. Of course the Japanese quickly surrendered.

In the Pacific, many Japanese soldiers did continue holding our for years. Growing up in the 1970;s it seemed that every year another Japanese long holdout in the Pacific would be discovered and finally convinced that the war really was over; that the Japanese had lost; and that Japanese cars were rapidly becoming the best selling motor vehicles in the United States. In many instances, such as for Hiroo Onoda in 1974, surrender was only enticed by locating the soldier's former commanding officer to appear and personally order a surrender.

However, Manchuria is most definitely not a tropical paradise where one can easily survive through a hunter-gatherer lifestyle. I expect the sheer logistics, and the sudden absence of supplies, were as much a factor in the surrender of the Kwangtung army as any of the other factors described.

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

In addition to @Semaphore's excellent answer about the timing of the surrender, the army defending Manchuria was of low quality. Despite having no hope of stopping the Soviets, they held on past the end.

In the late 30s, Manchuria was the front lines against the Soviets. The well-equipped Kwantung Army engaged in a series of escalating border conflicts with the Soviets culminating in total defeat at the Battle of Khalkhin Gol. The Soviet–Japanese Neutrality Pact sealed the deal and Manchuria became a quiet backwater.

With the Pacific War going badly, the Kwantung Army garrisoning Manchuria had been stripped to fight the Americans. Its 1st and 3rd Area Armies were intended for garrison duty with poorly trained troops and lacking heavy equipment.

However, as the war situation began to deteriorate for the Imperial Japanese Army on all fronts, the large, well-trained, and well-equipped Kwantung Army could no longer be held in strategic reserve. Many of its front line units were systematically stripped of their best units and equipment, which were sent south against the forces of the United States in the Pacific Islands or the Philippines. Other units were sent south into China for Operation Ichi-Go.

By 1945, the Kwantung Army consisted of a mere 713,000 personnel, divided into 31 infantry divisions, nine infantry brigades, two tank brigades, and one special purpose brigade. It also possessed 1,155 light tanks, 5,360 guns, and 1,800 aircraft. The quality of troops had fallen drastically, as all the best men and materiel were siphoned off for use in other theaters. These forces were replaced by militia, draft levies, reservists, and cannibalized smaller units, all equipped with woefully outdated equipment... The bulk of military equipment (artillery, tanks, aircraft) was developed in the 1930s, and very few of the soldiers had sufficient training or any real experience.

They were outnumbered 2-to-1 in men, 5-to-1 in artillery, 5-to-1 in tanks and 2-to-1 in aircraft. In addition, their equipment was woefully obsolete compared to Soviet equipment. Finally, unlike defending the cramped and rocky space of an island fortress, this would be a war of maneuver which by 1945 the Soviets excelled at.

The army began its surrender a day after the Emperor cryptically declared the war over. Individual units took a few days to make that decision.

The final commander in chief of the Kwantung Army, General Otozō Yamada, ordered a surrender on August 16, 1945, one day after Emperor Hirohito announced the surrender of Japan in a radio announcement. Some Japanese divisions refused to surrender, and combat continued for the next few days. Marshal Hata received the "ultimatum to surrender" from Soviet General Georgii Shelakhov in Harbin on August 18, 1945. He was one of the senior generals who agreed with the decision to surrender, and on August 19, 1945, Hata met with Marshal Aleksandr Vasilevsky, but asked that he be stripped of his rank of Field Marshal in atonement for the Army's failures in the war.