The in-basket has a limited capacity. It can hold only so much and than the contents are dumped to make room for more. Some of it is discarded. Nothing goes into the filing cabinet until it has been sorted in the in-basket. Use it or lose it.
Memories leave a trace in the brain that gradually fades with time.
The basis of forgetting is lack of use. You can combat this by repeating, rehearsing, using or practising whatever you want to remember. People with excellent memories have varied interests. There is a natural tendency to remember what we are interested in. We remember what we want to remember.