Continuity.

Why should we, the general public, care about these movies when clearly the people making it do not care about out intelligence.

If you are going to make a shared, connected universe, we expect it to have some consistency.

Example, sometimes you have to change an actor due to a death, or another Hollywood problem, and sometimes you can forgive the recast. Mark Ruffalo and Don Cheadle have been accepted as the better actors for their characters, but it still takes you out of the immersion if suddenly a characters appearance changes inbetween a movie.

Unless this is going to be a random stand alone movie: (Stop trying to make Super Hero movies that do not feature the main character in any way), it makes no sense that Leonardo Dicaprio would turn into Jared Leto after a few years (Seeing as this will be a prequel.)

And isn't the thing about the Joker that everyone likes is the fact that he doesn't have a solid origin story? Wouldn't this movie just ruin that whole mystique? Unless they make the main character an unreliable narrator.