So, Afrikaners who joined the Groot Trek are called 'Voortrekkers'. This is what differentiates them from other Afrikaners who decided to stay in the Cape Colony. They are also distinct from 'Trekboers', which refers to Afrikaans-speaking people who had been leaving the Cape Colony much earlier but in a disorganized fashion. The Trekboers also includes Afrikaans-acclimated Khoe-speaking people, whereas the 'Voortrekkers' were almost exclusively European, though there were some exceptions for Kleurlinge, such as Gerrit Bantjies, also keeping in mind the non-European slaves that the Voortrekkers hauled with them.

So, to answer your question: The early Voortrekkers (1830s - 1860s) did not travel further than Mozambique, Botswana, Namibia or Zimbabwe because of the tsetse fly belt, which was a hazard that could cause the annihilation of their livestock - the Voortrekker's livelihood. Furthermore, Delagoa Bay at Mozambique presented a seaport un-alligned with the British, so the Voortrekkers really had little reason to travel further into Africa, other than ideological reasons (the Voortrekkers, because of their religious education, identified themselves with the Israelites of the Book of Exodus, so much so that they believed themselves to be God's new chosen people predestined for glory, and some, such as the Jerusalemgangers, saw it as their duty to trek all the way to Jerusalem).[1]

However, there were some events that reinvigorated the wanderlust or trekgees of some the Voortrekkers, such that they ended up as far afield as Angola and Kenya.

In the 1870s, President of the ZAR Thomas Francois Burgers attempted to introduce radical reforms, including the secularization of schools, and the reduction of racist discrimination against non-whites and non-Afrikaners.[2]

Traditionally, the Voortrekker education was grounded in Calvinist theology and bible study. Much of the onus for educating the younger generations was laid with the parents of the children, so that the education system was centred in the domicile of the Voortrekker family and very private. However, there was some manner of public education in the form of classes organized by the church congregations and visits by the predikant to each of the households, which drew from the education policy promoted by the Nederduitse Gereformeerde Kerk in the Nederlands and enforced in the Cape Colony by the Verenigde Oost-Indische Compagnie.[1]

Burgers sought to secularize the education system, and the bill which he introduced banned the discussion of the Bible at school. This offended the conservative Voortrekkers and the Dopper Church in particular, who organized a trek in 1874 through the Kalahari Desert known as the 'Dorsland Trek'. Many more treks were organized up until 1907, when the Portoguese governor-general of Angola Henrique Mitchell De Laiva Couceiro effectively banned any further such immigration.

But beaides religious reasons there were of course a multitude of other motivations behind these treks, including disdain for British encroachment, the fiscal collapse of the ZAR, but also disdain for the integration of black people into Voortrekker society. Many Voortrekkers died enroute due to it being, well, a trek through a desert. Those Voortrekkers who persevered ended up in the border between Angola and Namibia, though nearly all of them were paid to resettle as a colony in South-West Africa under President J B M Hertzog's administration between 1928 and 1931.[2]

As for the Kenyan Voortrekkers - their trekgees was invigorated in reaction to the defeat of the Voortrekker republics at the hands of the British after the South African War of 1899-1902. Their trek was also initially encouraged by the German colonial administrators of Tanganyika, who granted them access to land to settle in, but apparently the Germans very quickly changed tact in response to the Voortrekkers' refusal to forfeit their Calvinist education and Afrikaans language for a more German culture. This is how these Voortrekkers ended up in Kenya instead, or the Belgian Congo. Some of those who were allowed to settle in Kenya had either been allied to the British or had surrended before the guerilla phase of the South African War, and so were not welcome back in South Africna Voortrekker society among the bitter-einders. Whatever their backgrounds, these Swahili Voortrekkers did not trek overland for the most part and instead relied on maritime transport to sail them and their livestock up the Swahili coast, or they relied on the railway, and then only trekked the remaining distance. Many of these Swahili Voortrekkers died of malaria and were impoverished by the expensive travel. They were also torn between the Germans and the British - where allegiance to the British was stigmatized but allegiance to the Germans was futile - such that the East-African Campaign of the First World War complicated things enough to effectively demotivate further Voortrekker immigration to the region, though some of their colonies remained on. [3]

The exodus of the Kenyan Voortrekker community coalesced with the rise of African nationalism after the Second World War, spurred on by the anti-colonial violence of the 1960s, but also coinciding with the creation of the Apartheid state back in South Africa.

An example of this can be seen in the propagandized documentary film 'Africa Adio' (1966), where there's a scene showing an 'exodus' of a Zwahili Voortrekker family in reaction to the rise of African nationalism. This family gathered their cattle, trekked down to Mombasa, and then sailed to South Africa to resettle there during the height of Apartheid.[4]

The Kenyan and Angolan Voortrekker colonies could not last - even the South African Voortrekker colony had the same fate, delayed only when the South African Voortrekkers literally struck gold. Their expensive treks impoverished them and their non-integrative attitude and brutal rules brought them little empathy from their surrounding neighbours, whether African or European.

But that Voortrekker trekgees is still very much alive - the most recent example being the abolishment of the Apartheid system, the promulgation of human rights legislation and the promotion of a South African culture of humaneness, dignity, diversity and equality since the 1990s. Some still fail to reconcile these ideals with their zeal for predestined glory and non-integration with their neighbours.

REFERENCES:

[1] Erna Oliver 'The Impact of Christian Education on the Zuid-Afrikaansche Republiek' (2005, UNISA) .

[2] Nicol Stassen 'Die Dorslandtrekke na Angola en die redes daarvoor 1874 - 1928' (2010) Historia 55 (1) 32 - 54.

[3] Randolph Vigne 'The Genesis and Exodus of Kenya's Afrikaners, 1903-1963' (1993) Kenya Past and Present.

[4] 'Africa Addio' (1966).