It's a return to Final Fantasy roots because it goes along with the original idea of a lighthearted adventure, set in medieval-times. The newer FF's were going in the sci-fi direction. Think of VII, with guns and Shinra and the cities, then in VIII with the gunblades and futuristic airships, and floating Gardens. Imagine if IX were to keep going from both of those and go even more futuristic. They didn't want to, and instead returned to their roots. Reintroducing crystals, four character parties, etc: medieval fantasy-style.

It's one of my favorite games in the series and it pains me when I see people that don't understand it.