Established memories have stronger synaptic connections between neurons that encode the memory. Also, these memories get ‘associated’ with other memories in the brain. These memories fade because

1. Over a period of time, the synaptic contacts become weaker unless we keep refreshing them by recollecting those memories. That is the brain mechanism to conserve space (available neurons). If something is not relevant, that tends to be forgotten so that more space would be available for something more important.
2. If these memories get disconnected partly or completely from other memories that were previously associated with it. That makes it hard to recall them. More the associations, easier to recall since there are multiple paths to the memory recall.
3. Aging also can weaken synaptic contacts.