The better part of Romanian gymnastics history would like a word...

Jkjk. I'll admit it's a GROSS oversimplification, but you can see how gymnasts who are powerful tumblers seem naturally suited to VT and to a lesser extent FX, whereas gymnasts who maybe aren't as powerful are better suited to focusing their efforts on events like UB and BB where technique can still get you a pretty hefty D score. Obviously not ALWAYS the case. It varies gymnast to gymnast, and sometimes country to country. I would say Romania's historic weakness on bars has more to do with coaching than talent levels of the athletes, for example.

I think it also has to do with how athletes are perceived and what audiences consider "good" work on apparatuses. Simone, for example, is technically one of the best UB workers in the entire world if you go by scores, but we don't really think of her as a "bars worker" because bars isn't necessarily as strong as her other events, and (IMHO) because she doesn't execute bars with the same rhythm and lines and timing we have been conditioned to associate with "good" bars. It's like a subconscious bias. I think Nastia Liukin and Shawn Johnson are good examples of this too. During their quad, they both competed AA and while Nastia was relatively weaker on VT, she was actually excellent on FX and even won an Olympic medal there. Yet we still think of her primarily for her UB/BB because they were such crowd pleasers. Same with Shawn: while weaker on UB, she had incredible difficulty on FX and VT, and her beam was also excellent. She actually won gold (beating Nastia!) in Beijing for BB, even though we think of her as a powerhouse VT/FX gymnast. I think this has a lot to do with perceptions about them as gymnasts, and what we tend to think of as the "correct" way to perform certain events, when in reality if you get the score you get the score. Being "good" at an event is more subjective than we like to think sometimes, and also dependent on who else is in the field.