Theravada couldn't have all the possible teachings, so it has only some of them. The teaching about the Buddha nature probably didn't seem necessary at the time of Buddha.
It was rather obvious that practitioners could follow the eightfold path and come to awakening (Arahantship & Buddhahood). It was mentioned in Pali suttas, e.g. in Maha-parinibbana sutta (see the last talk of Buddha, with Subhadda the Hermit).
So the need to introduce "Buddha nature" developed later. As dialog develops, it often happens that more ideas need to be articulated to help people understand well. That's why Mahayana teachings developed, and they continue to develop nowadays.
For example, when I explain Dharma, I often use language, ideas and pictures which didn't exist in Buddha's times. I speak about objects compared to their photographs, and it makes easier to understand, for example, Diamond sutra, Two truths and the like.
Thus the concept of Buddha nature developed with time, maybe as a pinnacle of explanation of why everything. Indeed, limited people have limited views and goals. We do something because we want something, we are attached or repulsed, and so on. But why would Buddha act? If he isn't limited, free from wishes, from attachments and repulsions?
If you say Buddha acts for the sake of other beings, then please recall that beings are illusory, and suffering is illusory...
It's not easy to understand: why do anything, what could be the goal, if there is no attachment, no limits...
So as understanding of illusory nature of delusions developed - helping people to practise efficiently - the need to understand what is natural developed too.
If you are interested to understand Buddha nature in the context of the three main philosophical schools of Mahayana, and their application to Zen practice, see:
Master Chi Chern. Immaculate Self-Nature