Originally Posted by The Joker
1. He was among one of the most prominent villains, especially after the 60's TV show utilized him three times and catapulted his status, just like with the Riddler.
2. Before the 60's show he only had like two appearances. After it he had many appearances.
After the 60's show (and before the DC Animated retcon) he had like 3 or 4 major appearances (not including cameos), which isn't many, and he still isn't a vital part of Batman's origin story.
That's because Joker doesn't have a definitive origin.