How did colonialism end in Southeast Asia? Why not earlier? Why did it occur earlier in some places than in others?

I would argue that official colonialism ended in Southeast Asia in the 1960s. The French left Vietnam, part of what was formerly Indochina, in the early-1960s and American forces moved in as early as 1964 to prevent the Communist Vietcong from seizing power in the country. President Kennedy expressed initial interest in preserving democracy in Vietnam. However, shortly after World War I, Ho Chi Minh directly appealed to President Woodrow Wilson in Paris to seek the United States's assistance in helping Vietnam become independent of France. His attempt to appeal to the U.S. president was unsuccessful.
What these early examples indicate is that the United States had no interest in interfering in the colonial interests of its Western allies. However, it had great interest in avoiding the spread of Communist regimes during the Cold War. Thus, colonialism ended in Indochina with the rise of Communist regimes, which were an attempt by native populations to determine their own futures, which meant rejecting the Western capitalist model that had exploited their people and resources for a century.
The United States's attempt to interfere in Vietnam, resulting in a series of carpet-bombings, as well as the abuses of villagers, could be interpreted as a form of neo-colonialism—that is, when a dominant economic and military power seeks to control a less powerful nation by influencing its policies and exploiting its resources without permanently occupying the nation. Some believe that the United States's hard diplomacy in the region helped lead to the rise of Pol Pot and the Khmer Rouge, though Cambodia was another nation, like Vietnam, seeking self-determination through Communism.
Before the end of European colonialism, Southeast Asia also awaited the end of Japanese colonialism, which occurred after the fall of Japan's militarist regime after World War II in 1945. Japan's colonial empire stretched from Korea (then unified) all the way down to Indonesia, which it colonized from 1942-1945. It had seized control of Taiwan as early as 1895.