The Afrikaners (Cape Dutch) Failed to Act

Furthermore, just as Boer national resolve seemed to be crumbling, that of the British Empire seemingly kicked in. The great hope of the republicans was a true people's war - a rising of the Cape Dutch against their colonial enforcers. This, their only realistic chance of success, might have swept the British out of southern Africa.

Many of the most ambitious guerrilla operations were designed to inspire such a rising, but it never came. In its absence, the Boer commandos were effectively confined to defensive operations and, although the guerrilla war lasted nearly four times as long as the conventional phase, many (if not most) Boers recognised the futility of their struggle against an empire with her immeasurable war potential and her seemingly limitless military capacity.

According to General Christiaan de Wet:

We knew, I need scarcely say, that humanly speaking ultimate victory for us was out of the question—that had been clear from the very beginning. For how could our diminutive army hope to stand against the overwhelming numbers at the enemy's command.

Source: The Boer War: Army, Nation and Empire