It's an intriguing question that is not asked often enough.

First, Dutch rule (including rule by the Dutch East India Company) had been present in parts of Indonesia for more than three centuries. The possibility of returning to pre-colonial political units was difficult to imagine. French rule in Indochina, by contrast, has been established mainly in the second half of the 19th century; the memory of previous political units was much stronger.

Second, the Dutch authorities had harnessed Indonesian diversity as a tool of control. Legally and administratively the colony was highly divided. The Dutch made use of the traditional aristocracies who were the strongest symbol of pre-war culture. For Indonesians who wanted justice and prosperity, sweeping away the old, divided order to create a single, united nation was critically important. By contrast, many Vietnamese nationalists regarded the cultures of Cambodia and Laos as backward. They feared that accommodating these conservative regions would compromise Vietnam's struggle for independence. Although Vietnam never quite surrender the idea that it had a natural hegemony over Laos and Cambodia, they were happy to jettison them for the sake of the revolution in Vietnam.