It Humanizes The Character
Tied to the point before, giving the Ace of Knaves an origin humanizes the character. It gives us perspective and perhaps even empathy to why he does what he does. And this will completely destroy his whole purpose of operating as an agent of chaos. There’s supposed to be no reason to what he does, except a lust for anarchy and seeing the world burn.
Before you argue how Tim Burton did a decent job of introducing Jack Napier in 1989’s Batman, ask yourself this: how much did you actually know about Jack at the end of the day? Not much. We knew he worked for Carl Grissom and killed the Waynes. Apart from when Bruce gave us an exposition-filled monologue about his prior convictions and all that though, he sounded like any other regular mobster. There was no sympathy for the devil, so to speak.
When you’re considered the living embodiment of pure evil, showing everyone how you had a loving family, kids, and a white picket fence doesn’t exactly add to your sterling reputation. It turns you into some form of Death Wish-inspired antihero instead, with a motive for your behavior. We’d prefer it if our Joker is kept insane, irrational, and as ambiguous as humanly possible. There’s no reason for us to identify with him, since he isn’t meant to be understood in the first place.
