It’s about cultural differences. Simone, Jordan and Eythora are from countries where it’s absolutely normal to refer to people using just their first name. It was raised recently that Gabby prefers to be known as Gabrielle, but I haven’t seen that be particularly adopted, either by fans or the media.Additionally, there is an issue with calling asian (or just Chinese?) gymnasts by their given name only? Because we talk about Simone or Jordan or Eythora all the time and aren't being asked to use their family name too (or instead).
I also think that Chuso is a good diminutive cuz it means I don't have to accidentally misspell her name but everyone knows who you are talking about.
Regarding Chusovitina, the Chuso/Chusa thing is purely phonetic. Chusa is how you say the first part of her name. It’s not an O versus A thing. Chusa is just what sounds right.