It’s not even a matter of powerful enough, it’s a technique that they had to learn. There was a frankly bizarre episode near the end of Clone Wars where Yoda had to go to a planet of jumping puzzles in order to learn how to manifest as a Force ghost, though I’m not sure if Qui Gon had to go there to learn it himself, and Anakin and Luke certainly didn’t.

I’ve always assumed that it involved dying in some state of total acceptance and willingness to let go of one’s physical body and become a purely Force-based being, but that’s more of my interpretation than something explicitly stated in canon.