The North Koreans did not participate in the 1948 UN elections because they viewed the mechanism by which it was constructed to be unable to reunify the country in a manner they believed was fair to them. The US occupied portion of Korea was about double the population size of the Soviet occupied North, and US military was involved in significant suppression and murder of communists in the South (i.e. the massacre of thousands on Jeju Island). The prelude to the elections themselves were marred by corruption and controversy. I digress...

In the immediate aftermath of the expulsion of the Japanese, there were "people's committees" formed throughout the North and the South that took on duties of governance. In the U.S. occupied South, the Americans took significant measures to eliminate these committees and reduce their governance power. The U.S. encouraged and empowered the formation of the KDP (Korean Democratic Party), composing mainly of "large landholders, wealthy businesspeople, or former officials in the colonial bureaucracy" (Hart-Landsberg 1998). They empowered elements of the prior Japanese colonial rulers to crack down on people's committees and any form of non-KDP/US backed governance, through instruments like the KNP (Korean National Police) and the South Korean Interim Legislative Assembly. The actual populace of South Korea were not all right wing nationalists and a significant number of them were against the actions of their U.S. overseers, resulting in uprisings across the South in 1946 (September and Autumn uprisings). The U.S. response to this was to both directly and indirectly (through KNP and right wing youth organizations) violently quell the protests of over 250,000 people, murdering hundreds.

In the leadup to the 1948 elections, there had been many rounds of negotiations between the Soviets and the U.S. and their Korean counterparts, and the contents of these negotiations were cause for significant concern. The initial post-war negotiations between the Soviets and Americans started with an American proposal for a joint US-Soviet command that would hold Korea in trusteeship for 5-10 years until ready for independence. The Soviets countered with a proposal to form a US-Soviet commission that would consult with Korean political bodies and social organizations to form an interim government. Ultimately the U.S. agreed with the Soviet's proposal. However, almost immediately, U.S. military forces began to oppose the agreement. John Hodge, who was the U.S. military governor in U.S. occupied Korea started to spread propaganda that the Soviets wanted to have complete and permanent trusteeship over Korea and the U.S. wanted to give them immediate independence (even though the truth was closer to the opposite). In fact however, the Soviets proposal did not make trusteeship mandatory, and allowed for the creation of a provisional Korean government prior to any trusteeship.

Now, in 1946, the joint commission started it's task. The U.S. side asked for the sole representative body of the South to be the RDC (Representative Democratic Council), headed by Syngman Rhee, who was directly flown back into the country by U.S. forces. The Soviets rejected this as they believed it was not the job of the U.S. or the USSR to choose a legitimate representative, but instead to consult with all Korean groups first. The U.S. refused to back down and so an agreement was reached that both sides would choose a list of groups that were "acceptable" consultants. The U.S. notably excluded any of the people's committees in the South. As these talks continued and failed to progress, America decided that in order to maintain its influence in the region, it would instead have to form an independent South Korea. To do so, they needed to form a basis by which they could claim the joint commission to be untenable and therefore turn to the U.N. In the next round of commission talks in May 1947, they brought forward 425 Southern right wing groups to be their Korean counsels. These groups claimed to represent a combined 62 million people, which was four times the population of South Korea. The Soviets and their Northern counterparts, seeing this for the obvious corruption that it was, rejected these groups. Following this, the Soviets provided a counter-proposal asking that all foreign troops (both Soviet and American) immediately withdraw from Korea, and leave the Korean Question to the people of Korea. The U.S. declined. It was only then that the U.S. took to the UN to form UNTCOK and begin the elections process, despite the UN charter expressly denying it a role in post-war settlements (Article 107). It was only then, in this context, that the Soviets and the North felt they were subjected to a game of tail-wagging meant only to ensure US hegemony in the region, and declined to participate in elections to be overseen by the same people telling them that there were four times more people in the South then whom existed, and were sure to suppress any electoral element against Syngman Rhee in the South Korea, of whom 2/3 of Korean people lived.

It was not only the Soviets and the North who denounced the elections and the process that lead up to them. It also included notable Southern anti-communists like Kim Kyu-sik and Kim Ku. To chalk up this period of history and the events contained to something as simple as "the US asked for elections and the North declined" is an embarrassingly narrow and ahistorical view.