Because some experiences have more of a profound effect than others. The brain has a wonderful filter system that helps us to keep what is relevant and dismiss what is irrelevant. For example, when driving, people can journey on automatic pilot, so to speak. They may forget the journey, but if there is something profoundly different, like a car accident or a flashing ambulance for example, whilst on route, they will remember.
Our brains are so complex that even if we are on 'automatic pilot', if something urgently needed recalling, we may even be able to remember parts of the information - the need for a police description of a car comes to mind as an example. It is possible that this sort of information can be remembered because we actively 'rack our brains' to find it.
Long term memory is often a product of some pleasureable or traumatic experience. Experience triggers chemical reactions in the brain which, sort of, etches to create pathways. The more the experience happens, the deeper the etching, if you like.
Memories, of course, can be subject to change, depending on our perception of reality. We often think that years ago, the sun was brighter and the grass was greener, but this might just be reminising and exaggerating the reality.
The truth is... what is your truth for this has an impact upon what and how you remember?
Hope this brief explanation provides added value to your question.