For one thing, one of the defining elements of The Joker is that his origin story is shrouded in mystery.
That being said, of all the DC heroes and villains to choose from to lead a standalone movie, giving The Joker an origin story is an odd move.
If I’m going to have a past, I prefer it to be multiple choice!”
In fact, one could argue that one of the reasons The Joker is such a popular super villain is because his past isn’t important; the mystery of where he came from helps make him more terrifying.