For roughly 6 million years, there appear to be multiple species of up-right-walking apes who also partly lived in trees and had roughly the same brain-size as chimps. It was a stable niche. Lucy was one of them.
Then new type of "ape" arose around a million years ago that relied ever more on tools and larger brains. The leading theory is that the climate started fluctuating heavily in Africa around that time, favoring adaptability over metabolic efficiency, and this is where human-ness branches off of ape-ness.