Why? I have always thought that backstory is prequel, because it happens before the main story... Or, rather, which are these differences?
I mean, it sort of is, but if there's no story to the back-story, then it's boring. Backstory would be sort of like all the early stuff a beginning writer is advised to cut out of the first draft, like the first three chapters or whatever, to get to the story. It's worldbuilding.
One of the short stories I wrote was to figure out how my villains became so villainous. I decided to write their first hit job. I did. I fleshed it out--who the victim was, how she ran from these villains, how they tracked her, and how they killed her and in so doing, how they became broken. The point of the story (for me) was to make them kill someone (and try to make it believable) so that they are more legitimately... pathetic and evil in the novel itself. I don't think I'll ever use that back-story as a story though, because even though it shows their first victim, it's boring. The reason it's boring is because she has no story of her own, no purpose--she's just on the run.
Backstory would be like a story of Luke Skywalker tending the moisture condensers or whatever at his uncle's farm, while his buddy Biggs is joining up with the resistance (the alliance)--and Luke is frustrated he can't go with Biggs. I don't know, it's possibly necessary for an author to figure out why Luke is so hell-bent to join the alliance in episode IV, and so maybe the author needs to world-build and write a prequel about Biggs. Maybe how he joined up is necessary to explain Luke's own frustration, but do we the reader really want to read a story about Luke hanging out with Biggs and shooting womp rats and throwing back beers after and then dragging himself out of bed the next day to fix condensers while he thinks about Biggs signing up? I mean, maybe. But it's just backstory, to my way of thinking. And a person could write endless back story.
YMMV.
ETA: What Benbenberi said.
ETA2: I think an arc is necessary. If Biggs had a character arc, then the prequel might work better. It'd end up being about him and his story--like was done in Solo.