OT takes place in the outer rim where there is less diversity, whereas the PT takes place closer to the center of the galaxy. The Empire was always two Sith Lords and a bunch of officers and clones. The Seperatists that supported them were diverse, but they weren't fighters, and were mainly wealthy and bought droids from the Nemoidans. Once the Clone army was built, they didn't need the Nemoids anymore. The Rebels were far more diverse, and that's shown in Return of the Jedi when you finally get a look at a large Rebel force. Tatooine is diverse because its in the outer rim and its where a lot of criminals and traders from different places came to ply their trade. Also shown off more in Return of the Jedi, though you get an idea when they get to Mos Eisley in Ep IV.
