Because for producer Michael E. Uslan the whole point of this film since its inception in the 70's was to get back to the roots of the character, and get him out of the shadow of the Adam West series.
I'm also familiar with that Tim Burton quote you're referring to. It reads to me more like a poorly thought out rebuttal than a factual statement. The unfortunate thing is many fans use that quote in order to discredit Burton's prep on the film, as if Warner Bros let a young filmmaker wander on the set, of what was the most expensive movie ever made at the time, without doing any research into what he was making. That notion is preposterous. There are multiple quotes, and articles that disprove Burton's statement.
Michael Uslan (producer): "I only let Tim see the original year of the Bob Kane/Bill Finger run, up until the time that Robin was introduced" (DVD)
Back in the late 1980s, Tim Burton took [Alan] Moore to lunch to pick his brains about making the first Batman movie. Moore's advice was "get Gotham City right".(guardian.co.uk int.2002)
Alan Moore (The Killing Joke writer): "I told them [Tim Burton and Samm Hamm] to make it dark and serious and exorcise the ghost of Adam West" (Ian Winterton Interview 2002)
".....it's another to relegate him to the background and make the movie entirely about the Joker." Another criticism that I'm very familiar with. I seem to be one of the few who has never had a problem with the amount of screen time Bruce Wayne/Batman have. Although, I do think that was a problem with the follow up film "Batman Returns". I never felt like the movie was all about the Joker either. You have Knox trying to uncover more about Batman, Vicki's trying to learn more about both Bruce Wayne and Batman, The Joker's seeking revenge against Batman and hates that he keeps upstaging him at every turn. I've always felt that this film revolves around Batman. On top of that Batman has an arc in the film, but the Joker doesn't.
The 89 film is one of those movies that feels like it was made for me. The things that people scold for were never a factor for me.