If you cut your skin it will regernate but after a brain injury ,you can't replace neurons. why is it so?what might be the evolutionary significance of this?

If you cut your skin it will regernate but after a brain injury ,you c

1.	If you cut your skin it will regernate but after a brain injury ,you can't replace neurons. why is it so?what might be the evolutionary significance of this?
Answer» If you cut your skin it will regernate but after a brain injury ,you can't replace neurons. why is it so?what might be the evolutionary significance of this?