According to freud why would certain desires and memories be hidden from conscious awareness

His theory is based on the "psychological unconscious." According to Freud, what is fundamental in the psychological order is the unconscious zone. The unconscious is dynamic and exerts a constant pressure on the conscious plane and, therefore, on the behavior of the person. This unconscious is constituted by instincts (everything congenitally given), but under the action of the outside world, the unconscious has undergone a transformation, which is consciousness, a force also capable of opposing the impulses of the unconscious.

Psychological Unconscious.