When Jedis die they can reappear as visions to the people they knew. Why is that a concept that needs further analysis?
Because when Jedi’s die they don’t usually show up as ghosts otherwise there would be nothing special about it. It was Qui-gon first. Then Obi-wan and then Yoda. and then Anakin.
No other Jedi was able to achieve becoming a force ghost. But it had to do with the special teachings of Qui-gon which he had learned and passed on his knowledge. Otherwise there would be millions of Force ghost Jedi floating about.
It has nothing to do with being a Jedi.