The central nervous system has an extremely delicate chemical environment where routine disturbances like inflammation would induce swelling and destruction of delicate neural pathways. In fact one of the primary roles of the blood-brain barrier is to ensure that any type of “growth” or “organ repair” is extremely retarded - any changes to cells happen very slowly compared to the rest of the body. This is one of the reasons brain damage is so slow to repair, the whole nervous system has a very prescribed way that it grows as a fetus and childhood/youth and then it puts the brakes on everything and only allows very small changes from then on.