While I am aware of the build-up prior to the campaign and do consider that a Soviet victory to be rather inevitable, the speed and scale of its success comparable to that of Operation Barbarossa, if not greater, is rather difficult to understand.
After all, the Germans also struck the Soviets at a time when they were at their weakest, but even their advance was fraught with various difficulties and logistical issues. The Soviets on the other hand, brushed off the 750,000 Japanese within a mere two weeks, and not only that, the Manchuria is full of terrain such as mountains and wetlands, which doesn't exactly make for an easy advance.
You have a few major factors at play:
The Soviets were heavily supported by logistics passing through Vladivostok from the US (under the guise of "Operation Mainpost"). This allowed for a much more rapid build up than might have otherwise occurred (indeed, the busiest time for "lend lease" through Pacific ports was after the defeat of Nazi Germany).
The Japanese forces in Manchuria were a shadow of what they had once been. The Kwantung Army in Manchuria had represented the best the Japanese had to offer in terms of Army units. However by the time of the Soviet invasion, 12 of the 19 Divisions from the Kwantung Army had been pulled to fight in the Pacific or to defend the Japanese homeland. The units sent to replace those removed divisions were very poorly equipped and trained, and some like the ill starred 30th Division had literally only existed for a matter of days before the Soviets invaded. This was further complicated by the fact the Manchukuo/local levies simply deserted/surrendered/opted not to fight.
The Soviet spearhead was frankly massive. In terms of MEN it was only around 2:1...but in terms of artillery it was a lot closer to 6:1, tanks 5:1, and Manchuria was virtually devoid of Japanese aviation.
There was a significant qualitative gap in equipment and men. As described the Japanese had many units that were barely stood up, let alone trained, but also they were equipped with anti-tank weapons that were woefully out of date (37 mm), very limited artillery and very obsolete tanks. The Soviets had T-34/85s, M4A2 76ws via lend lease, masses of heavy artillery, multiple rocket launch platforms, tens of thousands of trucks (mostly from American sources) largely manned by combat veterans from Europe. Unit for unit, the Soviets were better armed, and better prepared for the fight to come.
The battle also ended because the war ended. The Soviets did not fight and defeat all 750,000ish Japanese soldiers in Manchuria (Japanese records indicate about 40,000 causalities KIA/WIA, Soviets claim 83,000 KIA). Indeed, the war "ended" about a week into the invasion, but confusion over the surrender ensured pockets of Japanese forces either continued in error or opted to continue fighting.
Basically the battle went well because the Soviets were well equipped, well supported, and fighting an enemy that was unready, very depleted, poorly armed, and the fighting ended because the war did vs compete and total victory (given the Japanese tendency to fight to the death elsewhere, it would have certainly made for a very unpleasant cleanup operation had the war itself not ended).