Nicholson's Joker was simply too old. Joker should be around Batman's age and he should not, for God's sake, be the guy who murdered Bruce's parents. It was also hard to think of the Joker as anything other than Jack Nicholson wearing makeup. Jack had too much star power at that time. It would have been like casting Robert Redford as Superman instead of Christopher Reeve in 1978 (an idea that was floated by Warner Brothers executives at the time): "Look, it's Robert Redford in a Superman outfit!" Ledger, on the other hand, disappeared into the character. I never thought while watching him, "Here's Heath Ledger wearing makeup." He was only ever the Joker.