I'm probably stating the obvious, but Bars seems to be its own special animal - most different from all the rest.
Natural, strong tumblers and twisters on floor often find success translating to vault, beam...
Flexible athletes fare well with leaps, walkovers and such on floor/beam...
Strength and dance skills like presses and spins translate to both floor and beam..
Bars is so different - swinging, timing, wrist shifting, catching... The whole sensation and mastery of orienting and flipping around an object (the bars) in 360 instead of basically in a straight line across a horizontal surface...(floor, beam, largely vault). Sure there are some skills like presses than can aid in a cast handstand, but overall, just seems to be the event that stands out as requiring, on the whole, a completely different set of talents and skills. So it makes sense that it is most common to have Bars be the outlier that is either your strength or your weakness compared to the other three.
I'd say men's pommel horse holds this distinction in MAG - the event where being skilled on other events doesn't really translate (enough) - whole other set of techniques to swinging pommel.