For what reasons was the term manic-depression changed to bipolar disorder?

Good reasons: Here is a complete answer to your question: http://bipolar.About.Com/od/definingbipolardisorder/a/manic_depression_changes_names.Htm. Manic – depression was used to describe many types of conditions, not just those falling under the bipolar spectrum disorder umbrella. It was a stigmatizing label. The diagnostic criteria for bipolar disorders are sharply defined & the term is less pejorative.