Good question and a possible reason is that perhaps after 5 or so years Force Ghosts must leave this plane and move onto into the afterlife or “the netherworld of The Force” as Yoda called it in ROTS
This makes sense too. You’d think they couldn’t just appear forever and the length of time and who they are able to manifest themselves too might have some limitations.
In Heir To The Empire, Obi Wan appears to Luke for the last time with just such an explanation, having lingered too long already. Yoda and Anakin apparently never appeared to Luke again after the end of ROTJ.
We know that Disney will be taking cues from Legends. Who’s to say they didn’t take this one?