I mean why don't you call it "back". I dunno, I'm just stupid and think that everybody gets to somewhere folder by folder.
Up means it always goes one directory up, regardless of where you were before
Back means it returns to where you were last
They are two different things and really are not interchangable
If you just go folder by folder, they do the same thing until you try to go up from the place you started, at which point back is not helpful (not to mention that you need to store memory for every single directory you have been for the back feature to work)
Up is both simpler and more effective