Because the effects of the abuse can linger for life. A damaged individuel often has a kind of inbuilt self destruction mode that will undermine their every attempt to live a normal life and will always remind them of their past.
It's not always a matter of making a conscious decision to let go of the past. Certain events shape us and establish behavioural patterns that don' t become evident until much later. By the time a negative pattern is recognised, it may be so deeply ingrained that changing it involves breaking down everything you are and reconstructing yourself from the pieces. Some people go through years and years of behavioural therapy for a modicum of progress.
because some people believe that focusing on the past will help them figure out the future.