For the same reason as Dutch has disappeared from Indonesia, or Japanese from Taiwan. Except for English, all the colonial languages have gradually disappeared after the newly independent governments of the former colonies decided to stop using them in their educational systems, instead using their own languages. The last generation that grew up under colonial rule, which is also the last generation that knows the language eventually grows old and dies off, while the new generation never learns the language, leading to its disappearance.

English has thrived because it has become a global language due to American predominance. In fact, in all of the countries you mention, English is now the second language of choice.

Coming to Africa, the relationship of France to its former African colonies is very different from that between it and French Indochina. Due to geographical proximity as well as a large African diaspora in France, France is still a very important player in sub-saharan Africa. French businesses dominate the economies of the countries there. In fact, there are no less than 14 countries that use the CFA franc which was pegged to the French franc and is now pegged to the euro. There is even a specific term- "Françafrique", used for the French sphere of influence in Africa. All of this influence has ensured the role of the French language in Africa.

Vietnam, Cambodia and Laos had pre-existing monarchies, which the French made into protectorates, and which actually survived the colonial period until being overthrown by communist revolutionaries. In each case, there was an existing government, which already administered in Vietnamese, Laotian, or Khmer, and populations that already spoke those languages (although there are still minorities within each of those countries), and the French never dramatically altered those borders. In contrast, the French colonies in West and Equatorial Africa had borders that were set by French bureaucrats, and did not conform to the ethnic and linguistic situation on the ground. As a result, those countries retained French as an official language because it was common among the educated class, and it avoids the predominance of one language over another.