It’s because it’s traumatic that memories can be repressed, not in spite of. It’s not the same as saying you forget what you had for breakfast three days ago where your mind just never puts it in long term memory.

Memories of trauma don’t go through the normal process of how the brain handles everyday things but during the event your brain kind of glitches because of the sympathetic nervous system going haywire so the memory is not stored properly the body is focused on survival, not memory encoding and the memory bypasses the normal part and is heavily processed by the amygdala so the memory because visceral and attached to sensory memory instead of narrative.

The memory being so emotional and encoded differently is why we can have flashbacks to trauma and because our brain has never properly integrated them they aren’t retrievable like normal memories.

Anyone who was traumatized has the previous experience and can cause hyper recall of some events because of this encoding being so visceral and for others it causes them to be hard to recall. Both are super normal.

In those who can’t remember their memories much or at all it’s usually a function of dissociation. Dissociation can cause amnesia because if the mind is in a severe dissociative state during events it can become blocked out because the event is being processed by a mind that already has many functions running in a way that isn’t day to day.

This makes what are called dissociative barriers form in the brain to block out events so we don’t have to integrate them at all and it is our brain attempting to just live and not forcing the body to deal with anything. 

Some things around childhood trauma can be forgotten to as virtue of being really young. However most trauma is never forgotten as they are no longer there, the brain was changed by trauma like those who remember them the recall is what’s broken. So the brain is changed and the trauma being bad is why someone’s brain might forget.

Then structural dissociation is a factor in dissociative disorders, C-PTSD and BPD which means the structure of the brain was so altered by long term abuse as a child the wiring doesn’t properly connect the sections of the brain. Whole sections of the brain can have their development of size altered. And certain hormones or neurotransmitters were acting on hyperdrive for so long they are no longer function properly which can affect everything including memory recall.