[drited]

The book Brain Rules by John Medina is worth checking out for some suggestions on possible answers. I don't think science has definitive answers yet but if I remember correctly it's thought that the circuits involved in what the brain learned during the day are active during certain sleep stages which helps with consolidation of memory due to strengthening of connections involved. It's 10 years since I've read into this but I think the biology involved was that oligodendrocytes lay down thicker myelin on axons that are active, resulting in faster/stronger connections in the circuits involved. There's also inhibitory actions of established networks which have to be overcome by new circuits which takes sustained 'attack' over time. If the inhibition didn't happen the previously learned activities would turn noisy very quickly so there's a tradeoff between learning speed and maintenance of high fidelity signal for existing pathways in the brain. I guess the brain has evolved to accept new networks from actions that are repeated and thought about repeatedly because these generally are important whereas stuff that is thought about only once probably isn't.