There was a communist insurgency in South Korea before the start of the Korean War. Before it was crushed by the ROK army. The communists actually had wide spread popularity among the peasantry for their social justice and their resistance to the Japanese before suppressed by the South Korean Government. Source : [url]https://www.youtube.com/watch?v=3wJw8RYekEA#t=93[/url] The fundamental reason is the South Korean communist insurgency failed while the Viet Cong was much more successful is probably down to geography.

Look at the Geographical locations of South Korea and South Vietnam. South Vietnam has land borders with two countries Laos and Cambodia. The communists in Indo China were able to build a supply chain to replenish the insurgency there with a constant stream of men, supplies and ammo. The infamous ''Ho Chi Minh trails''. http://www.globalsecurity.org/military/world/vietnam/images/ho-chi-minh-trail.jpg To make thing worse for the anti-communists, both Laos and Cambodia were declared neutral zones. Only til the late very 60s and the 70s the anti-communists start plans to damage the trail after being fed up.

Meanwhile the communists in the South Korea didn't share this advantage, the communists were completely geographically isolated from their communist allies, since South Korea is apart of the Peninsula, the country is surrounded by all water in all direction expect at the North. Which is the 38th parallel which is heavily guarded by ROK forces and their allies. Supplying the communists by sea is too risky to actually work. In short the communists guerrilla in South had no means of getting supplied. Which directly means the communist insurgency was always doomed from the start. http://media.maps.com/magellan/Images/USAH022-H.gif

That is the easiest way I can explain it.