The Red Army was superior to the Kwantung Army in three main ways, all of which feed into each other. They had a superior doctrine, were superior technologically, and were superior numerically. The last two were both dictated by, and enabled the first.

The Red Army in the region, under Georgy Zhukov, used the ideas of Deep Battle formulated by Mikhail Tukhachevsky. While this had been discredited by Tukhachevsky's purge, Zhukov was willing to put it into practice in this case. This doctrine dictated broad-front attacks to open the enemy rear to strong combined-armed forces. These could then be used to destroy the enemy ability to fight, by occupying territory, or, as it was used in this case, by encircling the forward forces of the opposition. It would later be used successfully in WWII. In comparison, Japanese doctrine was static, with armour being used separately from infantry. Attacks were made using small groups of infiltrating infantry similar to the German stormtrooper groups of the later part of WWI. Defensively, the doctrine mandated a thin forward defence, where a defence-in-depth is the best way to deal with Deep Battle.

The Soviet doctrine relied on fast tanks, supported by motorised infantry. This resulted in production of the BT series of tanks, ancestors of the T-34. These tanks were fast, and well armed with a high-velocity 45mm gun. While they were lightly armoured, their guns had an effective range far higher than those used by the Japanese. Japanese tanks were much slower, with less effective guns. At the same time, they had the same low armour as their Soviet counterparts.The motorised infantry required large amounts of trucks, which allowed Zhukov to ship supplies forwards from his railheads with ease. The Japanese, who didn't have the same access to motorised transport, struggled with this. In the air, the contest was surprisingly even. The Zero and Ki-48 Oscar of WWII had not yet entered Japanese service. Instead, Ki-27 Nates took on I-16s. This was a more even contest than that with the tanks, but still in favour of the Soviets. The I-16 was more heavily armed, and armoured than its Japanese counterpart. The Ki-27 was more manoeuvrable, but the I-16 was faster in a dive. With the advantage of numbers and improved tactics, the Soviet Air Force was able to prevent the IJA from affecting the battle significantly.

Finally, as was common for the Soviets, they had numerical superiority. They had 3 rifle divisions, 2 armoured divisions, 2 motor rifle divisions and 2 tank brigades. They also had the support of 2 Mongolian cavalry divisions. At the point of attack, the Japanese had a single division. In general in war, the side with more forces wins - there's a Patton quote to the effect of "war being decided by the side who gets there first with the most".

The second part of your question has two answers. The first is doctrinal.The Japanese doctrine of 1938 had a clear family resemblance to that of 1905. They had taken the wrong lessons from 1905 and from WWI. The Sino-Japanese War had done nothing to disprove this doctrine. As a result, their doctrine was heavily focussed on infantry combat supported by light artillery. This was fine in 1905 and 1914, and worked against poorly led Chinese infantry. This led to Japanese army procurement being focussed on weaponry that fitted this plan. In 1905, these were the most modern things on the market - bolt action rifles, machine guns, light field artillery. In 1938, the same types of weapons were being bought. Armour was experimental, and used low-velocity guns for infantry support. Heavier artillery pieces were neglected - some of the heaviest guns used by the IJA in 1938 had been bought to deal with the forts of Port Arthur in 1905. However, the rest of the world had moved on. Heavy artillery, tanks and trucks were the most important things an army could have, and Japan had only the most basic of these.

Even had their doctrine been in favour of producing these, they would have run into trouble doing so. Japan didn't have the greatest heavy industry in 1938. When the country is only producing a limited amount of armour plate per year, and the navy wants most of it for building battleships, the army can't build tanks. There were not enough trained mechanics to support a large tank or motorised corps. Even the IJN's aircraft carriers only had just enough mechanics for their air groups - the most significant losses at Midway were in ground crew, not pilots. The Soviets had the heavy industry, and the technically skilled manpower to support the demands of modern war. Japan did not.