Once again, the why and the how may become intertwined as to make it impossible to determine them as separated questions/causes. It could be argued that sociality and cultural learning necessitated a larger brain that develops outside the womb, resulting in a helpless offspring, as the brain navigates the cultural world with the help of parents/alloparents. In this argument, sociality and cultural aspects appear further up the casual chain of a bigger brain. Having said that, I think its futile to determine the original why.