With respect to Korea, it started out partially occupied by communists, so North Korea was naturally going to be pushed in that direction. The South, backed by the United Nations and American aerial and nuclear supremacy, successfully fought off Northern attempts to extend communism to the entire peninsula in a brutal war. In Vietnam, the Southern government was weaker and there was no real prospect that its allies would invade North Vietnam. So this war was even more limited, and US nuclear saber-rattling was no longer credible given that the Soviets were approaching nuclear parity. Given North Vietnamese willingness to run appalling casualties and the decreasing casualty-tolerance of the American public, the outcome was a prolonged war which was won when North Vietnam invaded South Vietnam and the United States did nothing.

But Thailand was a different story. The indigenous communists were weak, and received only the most limited support from the People's Republic of China. Indeed, the communist insurgency barely crossed the threshold of civil war, according tot he Correlates of War Project. Moreover, there weren't huge NVA contingents to support the rebels as there were in Laos and Cambodia (both of which eventually saw communist takeovers). Since outsiders largely stayed out of the limited fighting in Thailand, the usual outcome occurred -- the much stronger government reduced the insurgents to insignificance, although they mounted a bit of a comeback at the beginning of the 1980s. So my answer comes down to military factors -- communists received huge external interventions in Korea, Vietnam, Cambodia, and Laos, but lacked such consistent and massive intervention in Thailand (and Burma, where the communist movement lasted longer before fragmenting into ethnic armies at the end of the 1980s).

What I don't know is why Thailand's government was able to repress its insurgents so quickly. After all, communists in Cuba started out heavily outmatched by government forces, but took control anyway -- and did so in the absence of substantial outside military intervention on their behalf.