Pretty much everything you'll read online says that it was because of the high risk of injury, but in my opinion that's only a small part of the story.
Not denying that risk was part of it, but it was the first move where a gymnast stood on the bar. I think a lot of gymnastics folks felt that it disrupted the overall flow of the routine, and it basically just wasn't what uneven bar routines were supposed to be about.
Things like this happen from time-to-time in gymnastics: someone finds something that is a bit "outside the lines", and then it gets banned. E.g. several years ago a gymnast did a floor routine wearing "cat-like" makeup (Maybe her music was from Cats? Can't remember) and then subsequently the FIG banned "theatrical makeup" in routines.
