Force Ghosts appear to those that they have a strong attachment to and have a strong attachment to them.
That's it. Continue reading if you want.
Maybe I should begin by explaining my fan credentials of Star Wars. There's not much there. As of this writing I've seen all but Rouge One. I've had the Blu-Ray since it came out but instead opted to watch When They Cry for the second time in a year. (Pretty amazing plot holes in that series.) I've only watched two seasons of Clone Wars. Excessively boring and an excessive amount of violence. I've opted out of ever watching Rebels. (So much yuri anime out there and it won't watch itself for the third of fifth time.)
To sum up I'm more of a casual fan. The movies are all that I need. Plus some YouTube videos. There's quite a few good ones. But the ones that got me going on this essay are about Force Ghosts. They'll spend from five to fifteen/twenty minutes telling us all about non-cannon Star Wars lore to explain this phenomenon.
In all those hours of videos there are two questions never asked and therefore never answered. Who are the people that become Force Ghosts and who do they appear to? To the best of my knowledge only four people have ever become Force Ghosts: Qui-Gon Jinn, Obi-Wan Kenobi, Yoda, and Anakin. There's a commonality to their appearances. Qui-Gon appeared to Obi-Wan. Then Obi-wan, Yoda, and Anakin appeared to Luke. (Maybe Leia, not too sure. Probably not.) Force Ghosts appear to those that they have a strong attachment to and have a strong attachment to them.
Only took me one title line of eight words to get to my point. Then three paragraphs of about 239 words and an hour's worth of research and typing to not explain my point.