What militants are you talking about? In South Korea, there were uprisings of leftist/communist peasants against the First Republic government of Syngman Rhee, but there weren't right wing or left wing militants/paramilitary groups. Rhee quickly crushed them, and at the outset of the war, murdered many leftist/communists, sometimes with the help of the US military.
If you're talking about the communist North and the capitalist South, what made it hard to compromise was Kim Il-Sung, Joseph Stalin, and Mao, and their desire to see a unified Korea under communist rule and the United States kicked out of Asia, and (for Mao), out of Chinese affairs.
This is an old thread, I realise, and you might not be interested in the topic any longer. I wrote a post on another thread that seems to be relevant to what you asked if you're still interested, I'll write some extracts below.
A key factor to understanding this within the context of south korea is that it's not a question of left/right, it wasn't an issue of ideology. Rather it was about pepole who'd collaborated with japanese wanting to avoid punishment vs everybody else. Everybody else did reach concensus, it became the KPR.
By the time American troops arrived, Koreans were deeply engaged in activity aimed at replacing the colonial regime and, they believed, establishing their independence. In mid-August Japanese Governor-General Abe Nobuyuki ... [ snipped text about fearing attacks against japanese in korea] ... approached Yo Un-hyong, a non-Communist Leftist whose demands for freedom of action were reluctantly accepted. Thus in Seoul Yo established the Committee for the Preparation of Korean Independence (CPKI) and began setting up a governmental framework for Korean self-rule. At the local level people’s committees sprouted up, often at the behest of the CPKI. These bodies frequently gained backing from native soldiers who had recently deserted the Imperial Japanese Army and armed youth groups (ch’iandae) and private armies that included thousands of prisoners – both political and criminal – freed by the CPKI.
Yo sought to recruit people of all political stripes, but most conservatives refused to cooperate, resulting in a Leftist-dominated organization. Korea in those days, according to one authoritative account, was ‘a maelstrom of old and new classes, political groups, and ideologies’. Four in five natives farmed for a livelihood, overwhelmingly as tenants. Japanese-directed economic development had added to the mix ‘an assortment of capitalists, white-collar professionals, [and] factory wage workers’. Wartime mobilization had produced increasing, often forced, internal migration and a draconian system of assimilation that sought to root out all vestiges of Korean culture, including names and language. While open resistance was impossible, Koreans accommodated their Japanese masters to varying degrees, producing resentments within the native population that promised to surface with liberation.
Broadly speaking, political groupings divided into Right and Left. Most Koreans with property and education had engaged in some collaboration with the Japanese, and they stood on the Right in opposing far-reaching change, such as comprehensive land reform or other instruments for redistributing wealth. Joining them were less educated, less prosperous people who had served in the colonial regime, including in the notorious police force, which was nearly 40 percent Korean.
On the Left were a variety of groups – intellectuals, peasants, workers, and students – who lay in wait to rise up against the government. Groupings were far from static. On the Left the Communists were the best organized and possessed some connection to the Soviet Union through its Seoul consulate, but they were neither dominant nor initially unwilling to work with others.
from "‘An Alliance Forged in Blood’: The American Occupation of Korea, the Korean War, and the US–South Korean Alliance, by William Stueck and Boram Yi, The Journal of Strategic Studies, Vol. 33, No. 2, 177–209, April 2010, pg 183
Basically it came down to the fact that the rest of the population would've wanted to hold the collaborators responsible for their deeds, and obviously the collaborators weren't going to have that.
The collaborators draped themselves in the "anti-communist" label in order to gain favour with the US military government, not necessarily because they believed in some ideology. The collaborators were given political power and power over the police, everybody else got labelled "a communist", and the US military government let them engage in a series of political suppressions, e.g. political suppression, violence, etc.
I copied some extracts from a relevant paper (Hodge Podge) in this post:
Frankly there was never going to be any real cooperation between the collaborators and the rest of the population because that would see the collaborators punished.