The biggest reason why the scientists believed that the humans originated in Europe and Asia was that the remains of the early humans were only found in these two continents at that time.
Africa was largely unexplored, especially when it came to the early hominids, sot he scientists very theorizing on what they had available, ans that was fossils from Europe and Asia, as well as cave art from these two continents.
When the Leakeys made the discovery of the Australopithecus, everything changed, and the theories about the human origin made a sudden twist.
It has to be taken in consideration though, that with the modern day technology, it turned out that the Australopithecus is not really possessing the right features to be classified as hominid, so it is becoming very debatable about the human origins again.
