Nature has wisely arranged it so that the memory and consciousness of most people is focused primarily in their current life. In fact, most of what is foremost in our consciousness pertains to the current day or week or even moment of life. It would be a great handicap if our subconscious or superconscious came flooding in all the time, and even a flood of conscious memories from the current life would render life unbearable and impractical. (Being able to access the superconscious selectively and at will is another matter that does not have the “flood” problem. And occasional memories and experiences of the remote past can also be OK and useful.)
There are vast realms of the unconscious which includes both the subconscious and the superconsciousness or soul. It is not correct that the brain can only access memories of the current life. Memories exist in the soul and for various reasons can sometimes filter down into the brain. The brain is a terminal that receives impressions from the physical world but also from the psychic and spiritual realms. These subtle realms are real dimensions and energies that under certain circumstances make their presence felt in the brain. External nature and super-nature are in continuity with each other and by evolution they become increasingly so.
It’s necessary to understand that consciousness and memory are not simply physical. They are present in the physical brain but that is not their origin. The origin is essentially and initially transcendental.