Level 9 bars can be a big sticking point for many gymnasts. The jump from a level 8 bar routine to a level 9 bar routine is profound. This is where a gymnast that is weak on bars may get stuck and you will see many gymnasts repeating level 8, sometimes multiple times because of it.
Bars is unlike any of the other events. All three of the other events use the same or similar movements to accomplish the same or similar skills, just on different surfaces. Bars it totally different. It requires a different set of strengths than the other events and you will often see this as a place of event specialization and that great bar workers may not be as strong on the other three events even at the highest level.
Think of Madison Kocian, one of the best bar workers in the world. She lacks the power needed to compete at the level of Simone Biles or Aly Raisman on events like floor and vault, but her clean lines and amazing flexibility, control and strength make her a wonder to watch on this event.
At the JO level you will see many gymnasts who are amazing tumblers and great at beam, but a great bar worker is harder to find and many girls struggle with bars at this point and beyond.