Personality disorders are thought to be more environmental (nurture, not nature) and so meds don't treat the real issue, only the symptoms, and therapy is needed to help the person learn to respond better to situations. Mood disorders seem fairly well established to be primarily biologically based, so meds address a very important part of the cause of symptoms, but therapy helps with associated behavioral challenges.

So BPD people can get some symptom relief from meds, especially when there are comorbid conditions (e.g. depression or bipolar), but therapy is really what it takes to help people avoid, reduce, and sometimes even eliminate recurring problems.