Why does brain metabolism not favor burning of fatty acids to provide energy?

Their oxidation in mitochondria produces a higher proportion of reactive oxygen species (ROS)—largely due to the increased FADH₂/NADH ratio and other inefficiencies in electron transport. 

Because neurons have relatively limited antioxidant defenses, the extra ROS generated by fat oxidation could cause damaging oxidative stress. 

This potential hazard has likely contributed to an evolutionary preference for using glucose as the primary fuel in the brain.

PMID: 23921897