There are three primary reasons for this. The first is the relatively small number of native groups that lived in the area. The area was relatively sparsely populated during the colonial period, both by Spanish settlers and aboriginals alike.

The second is immigration. In the post-independence era, the population was greatly enhanced via this immigration. Unlike other areas of South America, the majority of Argentina's population is descended from relatively recent immigration from Europe, specifically during the 19th and early 20th centuries. Although a not insignificant number of Argentinians have aboriginal blood, the majority of these are in the northern areas of the country closer to former native population centres. Indeed, most estimates put the percentage of mestizos, that is, mixed European/Amerindian blood individuals, at less than 10%.

Finally, as compared to Brazil, where a great number of Africans were introduced as slave labour, the area around the Rio de Plata, the main centre of population in the area, was not as well suited to the cash crops that other areas needed slave labour for, nor the temperature extremes and labour turnover (i.e. deaths from disease) that necessitated such. Overall investment in the area was relatively low for much of the colonial period as well.

Essentially, what made Argentina so attractive for European immigrants (land for farming and ranching due to low investment and lower population) helped contribute to this, as did the climate as compared to other South American countries.

Late in the train but I'd like to add /u/Legendarytubahero's comment to a similar question.

This is such a great question! Prior to the twentieth century, Argentina was not a populous nation. In this ensuing labor shortage, Buenos Aires became a key port for bringing the large numbers of slaves to their South American holdings. Most Africans were taken to work on estancias for agricultural purposes and to the mines in the Andes. As a result, in some places in the interior of the country, Africans and people of African descent made up more than fifty percent of the population in these areas according to Jonathan C. Brown. According the George Reid Andrews in his book The Afro-Argentines of Buenos Aires, 1800-1900, Buenos Aires’s population itself was a third black at the time of the revolution. The conundrum of where all the black people went has been a hot topic among Argentine historians for decades. And while it may be true that the nation that would one day become Argentina received proportionally smaller number of African slaves than other New World areas, to write off their influence and importance is not very accurate nor in line with recent developments in the historiography.

It was common to describe the drop in African lineages as a genocide in the mid-twentieth century because many historians believed that Afro-Argentine troops were used as “cannon fodder” in revolutionary armies, thus causing a significant drop in their population while serving to whiten the population. That is why Andrews’s book (mentioned above) is such a classic. In it, he not only destroyed the myth that Afro-Argentines were not important in Argentine development, he outlines a number of reasons why the population shifted so much. Famously, he also argues that although Afro-Argentines were drafted in disproportionate numbers, they were not killed in battle nearly as often as historians had previously believed; instead, they died of disease and deserted (thus calling into question the genocide accusations).

Instead, he points out that the flow of Africans was cut at the end of the colonial period and that African populations had a high infant mortality rate, low birthrates, and suffered through a number severe epidemics throughout the nineteenth century. This population-based thesis was taken a step further by M. Cristina de Liboreiro in her book No Hay Negros Argentinos? in which she explains that in such circumstances, African women intermarried with men of European and indigenous descent, allowing for significant assimilation into Argentine society.

Of course, these population patterns took time to run their course. In the mean time, the black population remained much more visible in Argentina. However, during the late 19th and early 20th centuries, huge numbers of Europeans poured into the country, rapidly and significantly altering the demographics of the nation. Immigration pushed the racial makeup of Argentina toward a lighter-skinned population. Simultaneously, as Andrews and others have convincingly demonstrated, part of the decline was an intentional “whitening” of the population during the late 19th century, which was pushed by positivist leaders who attempted to transform Argentina into a more European and “civilized” nation. In the process, they castigated their indigenous, Spanish, and African heritage in favor of the “progress” of Europe. This resulted in a significant white privilege among the elite and middle class. In popular culture, blacks were portrayed, according to Matthew Karush in his article “Blackness in Argentina: Jazz, Tango and Race Before Perón,” as the archetypical supporter of the demonized dictator Rosas and thus the enemies of progress. They were often the criminals and the idiots, playing second fiddle to strong white characters. These policies, both intentional and unintentional, took their toll on the African population. Sarmiento is even known to have said “En Buenos Aires, en veinte años más, será preciso ir al Brasil para verlos en toda la pureza de su raza” (In Buenos Aires in twenty years, it will be necessary to go to Brazil to see pure Africans).

Since Afro-Argentines was published, there have been many other journal articles and books that have sought to dispel the myth that Argentina was--and always was--a white nation. Afro-Argentines’ impact on Argentina, as you mention, is still reflected in Argentine culture through dance, music, genetics, political discourses, and literature. Additionally, Africans played a major role, albeit rarely discussed in English historical studies, in the construction of a new order after independence. Today, Argentine historians like Marvin Lewis, Dina Picotti, Oscar Chamosa, Carmen Bernard, and Mario Luis López continue to explore the legacy of this much “darker” Argentine past, if you’re interested in exploring more recent works.

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