At that time China killed each other over territory and China was not one nation yet so did India . But Siam was united and was one nation and the people who were loyal to the king.The Japanese was also united and the Japanese were loyal to the emperor unlike China and India who tried to murdered their own kings.
Siam was used as a buffer zone between the two competing colonialists, the British in the west side (Burma and India) and the French in the east side (Vietnam, Cambodia and Laos). It was the Lao king of who asked the French for protection from the control of Siam by becoming a French protectorate at that time. The French did not have much interest in Laos as they did in Vietnam.