There were aliens in the background, but none that would really give you a reason to care. The Mon Calamari were established as a group with agency in RotJ. They had their own fleet of ships of Mon Calamari design, crewed by Mon Calamari - they weren't just part of the Imperial melting pot before they rebelled. And races like Wookies and Twi'leks that don't have this are still distinctive and interesting in a way that the DT's various potato people aren't. Nobody's a fan of the Abednedo the way they might be fans of Twi'leks, the Klingons or the Turians.