Well, just because there wasn’t a comprehensive plan for the continent doesn’t mean that the U.S. didn’t give aid. Loans and credits were offered to governments on an individual basis. Taiwan received over a billion dollars (in addition to the military aid the Chinese Nationalists had received, which was on the order of 4 billion by 1949; close to a third of all money spent in the Marshall Plan!), the Philippines close to eight hundred million, Indonesia around two hundred million, and Japan over two billion. But you might wonder why it was so piecemeal and so varied.

Part of that is simply down to Cold War priorities that were set by leaders in Washington. People such as Secretary of State Dean Acheson, George Kennan (author of the Long Telegram, which helped develop the policy of Containment) and others saw Europe as the fundamental battlefield of the Cold War because it was simply the wealthiest and most powerful area to contest between the US and the USSR. Throughout 1946 and 1947, there was a growing fear in Washington that communist parties in Western Europe would gain ground electorally if the collective European economy continued to suffer. The Marshall Plan was a corrective to that, along with other policies such as military aid to Greece and Turkey. While there were humanitarian concerns undergirding the Marshall Plan, the core was geopolitical: ensuring European prosperity would keep Western Europe from turning toward communist parties as a solution. Asia was a lower priority in most ways, though of course there was a vocal China lobby in the United States, and Japan was generally considered an industrial center and part of the U.S. security bulwark against the Soviet Union. Kennan in many ways was nevertheless resistant to the idea that the U.S. should take on obligations everywhere (John Lewis Gaddis’ Strategies of Containment lays out Kennan’s worldview nicely).

There’s of course a whole other reason. Of the Southeast Asian countries you just listed, only one of them was independent at the time of the Marshall Plan: the Philippines. Everywhere else was under colonial rule, and foreign policy traditionally held that the metropole was responsible for the finances of a colony. Indonesia became independent in 1949, partly at the prompting of the United States (which threatened to withhold Marshall Plan aid to the Netherlands if they continued trying to maintain their rule), but Indochina didn’t become independent until 1954 and Singapore and Malaysia until 1957. This accounts for a much more eclectic policy, especially considering that the U.S. opposes some national wars of liberation and bankrolls the colonizer to fight (France and Indochina) but then opposes the colonizer elsewhere, or simply lets the colonial power set policy (Malaysia and the British).

If you’re interested in U.S. policy during this period, I’d look at either Strategies of Containment, A Preponderance of Power by Melvyn Leffler, or Michael J. Hogan’s The Marshall Plan.