Lucas was reluctant to show off a lot of aliens in the original trilogy. If you rewatch those films, most of the masked aliens are shown in carefully or dimly lit scenes; the Cantina, Dagobah, the Carbon Freeze Chamber and Jabba's Palace.
Lucas was concerned that audiences would find the masks hokey, so he cut away from them or just showed them on screen very quickly.
Prosthetics and animatronic mask techniques have greatly improved since 1979 and we've seen some amazing alien creature work in the prequel trilogy. It's a massive galaxy and seeing human after human is becoming a bit naff. It seems to be a Disney thing. It's far more economical to cast a bunch of human extras than it is to create an array of convincing alien masks.
That's part of the problem with Disney. They're not interested in spending the big bucks to make Star Wars look grand and feel elaborate, which is what it should be. This is why they keep hiring mostly low tier directors and writers and why these streaming shows end up looking and feeling like fan fiction.