The official reason was to "tie-in" both of the Port Orleans resorts and give them a unified feel.
Rumors have flown for years about whether this is the real reason or not. Many people feel that the name was changed because the word "Dixie" has all sorts of negative connotations regarding racism, slavery, etc. Personally, I feel that's a load of bilge, but given how sensitive people are these days to words, and how often people decide that a word means something and then foist that belief on everybody around them, it's certainly not impossible that Disney might have changed the name strictly to avoid offending a small group of people who think the word "Dixie" is evil in some way.