Not an evolutionary biologist but my guess would be it has something to do with niches in an ecosystem and lack of evolutionary pressure to change in an environ

Not an evolutionary biologist but my guess would be it has something to do with niches in an ecosystem and lack of evolutionary pressure to change in an environment in which you have reached some equilibrium. Chimps for whatever reason were able to satisfy their biological imperatives within their given environment early on and have never needed to change. The human line, not so much. I guess the way I see it is that some mutation in early humans (perhaps selected for by the environment) created a priority on brain development. I'm sure there were many dead-end branches along the way to us that we haven't noticed. We just look back and see a straight line from chimps to humans. Perspective is key here. Of course, I could be completely wrong :)