French Indochina had 5 subdivision: Cambodia, Laos, Annam, Tonkin, and Cochinchina. Khmer were the majority in Cambodia, Laotians in Laos, and Vietnamese in the other three. When they won their independence in 1954, the three Vietnamese subdivisions unified (sort of because they were soon divided into North and South Vietnam) as Vietnam. There was no region where Chams were the majority. Even today, the total number of Chams is well below 1 million (closer to around 400,000) and they are split somewhat evenly between Cambodia and Vietnam with Cambodia having a slightly larger proportion. Even then, the population of Cambodia is about 15.5 million and Vietnam's is about 91.7 million people. The Cham people are a vast minority wherever they live, so it didn't make sense to give them their own subdivision (like the Alawites or Druze in Syria) that could have potentially won independence.