Answer:
Because the other countries were just gaining its independence from the Eropean colonies and were trying to establish themselves.
Explanation:
ONly the Phillipines and Malasya as South East Asian countries joined SEATO, other countries that remained observers but never joined due to their recent independence from Indo-France were Vietnam, Laos and cambodia, they wanted to join but didn´t have the political strength to do so. The countries that joined like England, Australia, the United States, saw southeast asia as key for their foreign policies and if they fell under communists threats that would affect largely their interests.
