>>123029448She was originally kind of a mistake. Due to the Wonder Woman writer (and it was the same guy for decades) making "impossible stories," the Teen Titans writers didn't realize they were making a mistake.
In the "impossible stories," Wonder Woman, Wonder Girl, and Wonder Tot would team up for adventures. They were all the same person though. The first story was made by Hippolyta literally editing film footage (don't ask how they had cameras) of Diana at different ages to make it look like she was with different people.
By the time Brave and the Bold was happening, that "this is a fake story" disclaimer kept getting smaller and smaller. So when "Wonder Girl" first appeared in Teen Titans, it was accidentally Diana. Later on, they created "Donna" to differentiate them. Donna was only in like eight issues of the original Wonder Woman title.
>>123029469That's more because writers (especially Byrne in the 1990s that brought Donna into the book) ejected all the supporting characters that were already around and made his own OC's.
