Well where the Dutch actively colonized, and not just politically controlled, they spread their language. For South Africa, Afrikaans is spoken by millions it basically is Dutch with large amount of English and other lingual influences. The debate on whether or not Afrikaans and Flemish are dialects or distinct languages is whole another long debates.

But, you are right in that for the vast majority of their colonies, their language did not spread enough to leave a significant impact upon those countries as we see with other European empires. The Dutch, first and foremost, established their colonies in South Africa, India, Indonesia and elsewhere as a means to facilitate trade. The Dutch were remarkably good traders and in general they only aimed for political control as necessary in order to support their trading empire. Unlike the Spanish or Portuguese, the Dutch were no where near as set upon establishing political, social, and religious control over their colonies (which is why the Japanese eventually granted them exclusive access to trade). By the time the Dutch were forced out of their colonies, the institutions and the amount of control exercised over Indonesia was far beneath what the British had achieved in India and their culture, language and religion were not as far spread as that of other European nations.

I know mostly why it didn't spread in Indonesia at least, though I can't speak for the other colonies.

Adding to all the comments below, around the VOC Era, in Indonesia at least, there WERE actually Dutch schools, that teach in the Dutch language, most of these schools were usually for elite and high-classed people, so plenty of the population who weren't able to get in just couldn't speak the language.

Like /u/Czone said, the Dutch prevented their language from spreading because they're afraid it would make the population too smart. Adding to that, when the Japanese invaded, they denied many Indonesians from using Dutch words, allowing only Asian-based languages, like Malay, Indonesian, and Japanese to prosper, so that was a factor.

As an Indonesian, I also wouldn't say that the Dutch language itself spread, but it certainly had a huge effect on our language. Plenty of words in Indonesia had Dutch-based origins. The Indonesian word for cigarettes for example, Rokok, had influences from the Dutch word Roken.

Lots of people answering here but no sources. I'm basing my post here on Gaston Dorren's article in Onze Taal magazine, 2015/4. It's not academic, but it's okay.

One notion Dorren dispels is that colonial powers were effective in spreading their languages. For the most part, they actually weren't (although every country's got a different story). The colonial education systems only taught the colonial language to a small subset of the population, that would later go on to form a new elite, while the rest of the country spoke their local languages. Since post-colonial borders often meant a highly diverse country, a single government language was needed. The language of the former colonial power was a popular choice, since it was already mastered by the elite, and politically neutral for internal purposes.

Indonesia is different. There actually was a Dutch-speaking elite, after the war it didn't rise to become a national language. The reasons for this were that A) another language was available to serve as a neutral government language, namely a version of Malay that had been a trade language for centuries, and B) Dutch was not politically expedient.

It's important to consider how the Indonesian national identity worked. Indonesia is an incredibly diverse country, and the Indonesian national identity was 'created' in the 1920's and 1930's almost purely in opposition to the Dutch colonial overlords. It stressed unity of the archipelago over the divide-and-conquer mentality of the Dutch. This also meant that no native language could have the upper hand, especially not Javanese, the most numerous language in terms of native speakers. Dutch was of course out of the question, so they went with their version of Malay.

Indonesian de-colonisation was very messy, and the new Indonesian government and the Dutch still had arguments over exactly which groups were to be part of Indonesia. Moluccans and Papuans were two groups that opposed the new Indonesian government, and they had the support of the Dutch government. This made the Dutch language even less politically expedient; it caused a movement to remove Dutch words from the Indonesian (ie. Indonesian Malay) language, and alter its spelling into one less obviously influenced by Dutch; Soeharto became Suharto, for instance.

According Regerings Reglement 1854, the society in East Indies was divided into several classes based on rigid racial system with Europeans (mostly Dutch) at the top level. They weren't allowed to intermingle, but still allowed to interact in trade, official business, etc. The Wijkenstelsel that divided towns and cities into districts and settlements based on race and ethnicity was a direct implementation of this regulation. And before the implementation of Ethical Politics only the Europeans and a handful of privileged Chinese had access to education, which of course used Dutch as official language.

But in early 20th century education were opened to privileged indigenous people too (local aristocrats, civil servants, local landlords, etc) and it doesn't take long for the educated youth to use Dutch as a lingua franca among themselves. The use of Dutch and modern (Western) lifestyle was what separated the educated class and common people. Most people at the time were uneducated or educated outside the European system, so large portion of the indigenous society didn't speak Dutch and simply didn't have the means to learn it. And before the use of Dutch became widespread, the colonial administration was replaced by Japanese occupation (in which Dutch was forbidden to use, and the use of Indonesian and Japanese was encouraged) and period of national revolution that made the use of Dutch unpopular.

And in my view, the colonial administration never had the intention to popularize the Dutch language or assimilate the whole indigenous population into the Dutch culture because simply they didn't want to. And they really did.