The central nervous system and the peripheral nervous system differ in the way the nerves regenerate following injury. What is the reason for this difference?
It comes down to the differences in the way that the fibers are formed.
For many reasons, the repair in the central nervous system is prevented by factors that prevent remyelination.
The nerve fibers that are not myelinated have a better chance of regeneration and repair because of their basement membranes that act like sign posts.
There are other factors involved including age and general health.
Here is a more complex description: