Why would you not want her as oracle, a role that pushed from just one of the many interchangeable batfamily vigilantes to her own distinct and incredibly influential being.
That is a very difficult question to answer without sounding dismissive of readers who found inspiration in the Oracle character, but I'll try to do it while not being offensive (uh-oh).
First, Barbara
as Batgirl is not 'just one of the many interchangeable Batfamily vigilantes'. For twenty years, there was Batman, Robin (Dick Grayson) and the Barbara Gordon Batgirl. There were three Batbooks- Batman, Detective, and The Batman Family, which starred Batgirl and Robin (so when some assert that she never even had her own book, well, that's not quite true). Other than Wonder Woman, you can argue whether Batgirl or Supergirl was the second most popular female hero in all of comics at that time. And with her many team ups with and cover appearances on Superman during this period, my guess is that TPTB at DC thought- at least when it came to boosting sales- that it was Batgirl.
So, as far as history, prominence, and influence, Barbara as Batgirl is not just another interchangeable Batfamily vigilante.
Second, I absolutely despise 'The Killing Joke'. Not just for the appalling treatment of Barbara, but also for Batman. When, after the unspeakable abuse of Barbara and Jim Gordon (two of Batman's greatest friends and allies) Batman embraces and laughs with the Joker at the end, he is revealed as a fellow sociopath. Everyone else are just disposable characters in their little play. So, the Oracle character is a constant reminder of a story that I consider a grotesque abuse of not one, but two characters that I love dearly.
Third, I think that Barbara
as Batgirl presents greater dramatic story possibilities than any supporting character in the Batuniverse, because of her father's relationship with Batman. Paul Dini certainly recognized and exploited this to the fullest in the great, emotional 'Over the Edge' episode of the animated series. Speaking of B:TAS, I think that those guys- particularly Dini- deserve great credit for the rehabilitation of Barbara as Batgirl. For a show that was famous for using the very best of Batman storytelling, 'The Killing Joke', rather conspicuously, never happened, as Barbara became a perfectly ambulatory Police Commissioner in Batman Beyond.
Before the internet, one had to buy magazines like Comics Scene, Wild Cartoon Kingdom (really), Animation Magazine and Hero for information on properties like B:TAS. I remember an interview in one where Paul Dini was asked 'Why Batgirl?' after it had been revealed that young Barbara would eventually become Batgirl on the show. He answered 'Because we love Batgirl', which I found so gratifying, because it was actually an enormously brave thing to say at the time (in the so serious '90's, perhaps the height of grim/dark, Batgirl- a character created in coordination with the then-reviled '66 Batman tv series- seemed to be the very symbol of 'camp'). I think Paul Dini may feel the same way about 'The Killing Joke' as I do.
Lastly, since the beginning, a great part of the appeal of superhero comics has been the dynamic depiction of human anatomy in very colorful costumes.
And yeah, I want that for Barbara on the big screen.
If she must be Oracle, I would hope that would happen after a very healthy screen life as Batgirl.
At the very least, for dramatic storytelling purposes.
Starting her off as Oracle would be like- no, worse than- starting the Star Wars films after Luke has fought Darth Vader and lost his hand.