There's still some truth to the thought that African languages were suppressed by slave owners, whether through mandatory conversion to Christianity with services in European languages or through direct command and punishment. In at least some cases, African languages had to be spoken in private or with caution, which is already a situation that makes it difficult for them to thrive.

But the common perception that Africans taken in the Atlantic slave trade spoke a huge variety of languages and were isolated from others who spoke their languages in a deliberate 'divide and conquer' strategy is now thought to be exaggerated or simply incorrect by many historians studying the Atlantic world. Gwendolyn Midlo Hall, John Thornton, and many other scholars have observed that in fact there were strong clusterings of particular West and Central African groups speaking the same or closely related languages in various eras of the Atlantic slave trade, and that many West Africans in particular spoke two or more languages moderately well in their home communities.

So how to explain the seeming disappearance of their languages in the Americas? First off, by noting that in fact those languages did not disappear entirely. Gullah/Geechee culture in South Carolina and Georgia in the US is probably the best known example of the survival of African languages in a creole, but there are other creole languages of a similar sort throughout the New World, often associated with primarily African-descent (sometimes with Native American influences) communities that for whatever reason were a bit isolated from white-dominated centers of population. There are also religious practices and ceremonies throughout the New World whose practicioners speak largely or partially in one or more African languages or a creole during rituals, such as candomble in Cuba.

More importantly, perhaps, is that the OP and anyone else asking this question needs to consider the time frame they're addressing, both in terms of the difference between a 17th/18th Century Atlantic world and the present and in terms of what happens to languages generally over longer time spans. In terms of thinking about the 17th/18th Century Atlantic world, the answer to the OP's question is quite different than it is in the context of 2019: in the 1600s and 1700s and into the 1800s in the Americas, African languages were spoken with some frequency. There's considerable evidence, for example, that the primary participants in the Haitian Revolution (outside of the leadership) were coordinating their activities in several African languages. James Sweet's book on Domingos Alvares and his earlier book Recreating Africa describe a number of situations in which slaves in Brazil, particularly first-generation captives, spoke to one another in West and Central African languages. That was likely true for most of that long period of two or more centuries--that there were groups of slaves and freedpeople who spoke African languages or creoles based significantly on one or more African languages.

So the OP's question is then: why did this situation not persist? And that's where we need a comparative recognition of the total linguistic history of the Americas, which helps underscore that very few languages spoken in earlier eras of settlement and colonization have persisted. In the original thirteen colonies of British North America, for example, there were significant numbers of people speaking German or Dutch in Pennsylvania and New York. If one asks, "Why do they not speak German in Germantown Philadelphia now?" the answer seems obvious: the early German speakers did not live in a highly isolated community, they did not remain in place between then and now, and most importantly, that was a long long time ago. For a language to be spoken in a continuous way within a territory that has a major, dominant language associated with government, political power and education, the people speaking that language typically need to be physically and culturally isolated from the communities around them, they need to have civic institutions of their own (including control over education in particular), they need to have something of an ideological commitment to maintaining their sense of separation, and so on. Which in fact is why some Amish today do speak a dialect of German.

The point is that this is a somewhat unusual circumstance in general: most 18th and 19th Century immigrants to the Americas from Europe kept a working knowledge of their natal languages for no more than three generations, though of course there were unusual households that worked hard to hold on to that knowledge on an individual basis and there were a few notable exceptions (such as Yiddish). Where that process of loss certainly accelerated is in late 19th Century urbanization and in the rise of public education through much of the Americas, and both of those processes particularly made the survival of non-dominant languages unlikely for communities living under some form of racial domination. To some extent, the OP's question is not that different than asking, "Why don't more people in various regions in Europe speak languages like Breton, Catalan and so on that they spoke more commonly in 1750?" The answer is on some level the same: because governments discouraged or suppressed such languages, and because many socioeconomic changes dispersed tight-knit communities and encouraged forms of internal migration. There is nothing special in this sense about the African languages and African-influenced creoles spoken in the Americas in 1750 or 1800: they aren't spoken today for the same reason that initially tight-knit families and communities of Swedish immigrants in Chicago and Minnesota who arrived in the mid-19th Century stopped speaking Swedish by the early 20th Century.