SEATO is the Southeast Asia Treaty Organization that was formed to prevent communism from gaining ground in the Asia region. The colonialism in southern Asia helps to explain why there were only two countries in the Organization because the Philippines had close ties with the US and Thailand had concerns over a potential Chinese communist subversion.
The other countries were countries that colonized the area and had interest in the region and in the organization, Australia and New Zealand had interests in Asian Affairs because it’s proximity, Great Britain and France had colonies in the region, Pakistan wanted support against India, and the US joined the organization to fight against communism and saw the organization as a Cold War policy.