I don't see this as a big deal. They're around during the OT period, just not seen onscreen.
It's been happening all along. Some of the ANH cantina aliens did not reappear in ROTJ (Abyssin, Gotal, Givin, Morseerian). Gands (i.e. Zuckuss) didn't reappear in ROTJ. And Ishi Tib appeared in TPM and AOTC but not ROTS, then pop up in ROTJ.