Neuron repair inhibition in the CNS
I almost got it right. The consensus as of 2018 appears to be that it's a Faustian pact between providing the CNS with our unique mental powers at the cost of poor damage repair. This is not an issue in the PNS where damaged neuron repair is, in the main, impressive.

Quote (1): Principles of Neural Science 5th Ed (2013) (Eric R Kandel et al)
“ ... the regenerative capacities in the peripheral nervous system are impressive.
In contrast, in the central nervous system regeneration after injury is poor.”
“Is it the ability of peripheral axons to regenerate that is unusual, or the inability of central axons to do so? It is in fact the latter.”
“Maintaining constancy in the face of small perturbations in connectivity may therefore have the unavoidable consequence of limiting the ability of central connections to regenerate in response to injury. In this view our limited regenerative capacity is the Faustian biological bargain we have made for the possession of many precisely wired circuits that underlie our superior intellectual powers.”

Quote (2): Transl Psychiatry (2017) 7, e1214; doi:10.1038/tp.2017.170 www.nature.com/tp
“Reticulon 4 receptor (RTN4R) [aka NgR and NgR1] plays an essential role in regulating axonal regeneration and plasticity in the central nervous system ...”

Quote (3): A.R. Walmsley, D. Bourikas And A.K. Mir, 15 - Myelin Neutralization For Spinal Cord Injury And Stroke, In CNS Regeneration (Second Edition 2008)
“ ... the environment of the CNS is highly inhibitory to axonal regeneration and plasticity. In recent years, great advances have been made in our understanding of the molecular components contributing to the inhibitory environment of the CNS. The majority of these have been identified as membrane proteins present in CNS myelin, such as myelin-associated glycoprotein, Nogo-A and oligodendrocyte-myelin glycoprotein, that inhibit neurite outgrowth by binding to the multimeric Nogo-66 receptor (NgR1) complex on the neuron ...”

Quote (4): https://en.wikipedia.org/wiki/Reticulon_4_receptor

Reticulon 4 receptor (RTN4R) also known as Nogo-66 Receptor (NgR) or Nogo receptor 1 is a protein which in humans is encoded by the RTN4R gene. This gene encodes the receptor for reticulon 4, oligodendrocyte myelin glycoprotein and myelin-associated glycoprotein. This receptor mediates axonal growth inhibition and may play a role in regulating axonal regeneration and plasticity in the adult central nervous system.