Why does it take so long for the human brain to develop from an evolutionary point of view?
We can say that brain and our nerve system is the first system in embryo that starts to develop and as you said this system is continuing to develop until after birth. 
So here is a question that why our brain don't develop completely before the birth ?  Evolution has gone so far as to limit the development of the brain in the human embryonic phase and to allow it to continue into the postnatal phase. This helps the infant to be born and ease the birth both for mother and the new born because if the brain had fully grown, the size of the head would have made problem in birth. Now, after birth, the brain continues to grow and develop majorly between ages 2-3 and becomes more mature after that . The ability of the brain to grow over the years gives us the ability to adapt to different environments , learnings and new issues and other capabilities That happens with the subsequent creation and pruning of dendritic spines.