Because an infant's brain is still developing while an adult has passed that stage.

The infant brain has better plasticity than the adult brains. The regeneration of brain and vessel in the damaged region occurs in both infants and adults, which means the brain can also regrow in adult to some extend. However, the ability of regeneration seems to be higher in infants. Additionally, infant brain has more "clean space" to establish the neuronal circuit for functional restoration.

@hao jiang Thank you.

Can you also relate this to the fact that infants have more neurons than adults?

Aliya Mirza Yes, that's a good point. More neurons and more synapitc connections.

Article Brain Plasticity and Behaviour in the Developing Brain