Ben Affleck's Batman: What Went Wrong?
2. Matt Reeves Wanted Something Very Different
While Matt Reeves largely played coy with regards to who would don the cape and cowl in his Batman movie - alongside persistent rumours that Affleck wouldn't be in the movie - it did become increasingly clear that he had his own vision for the character.
That's to be expected, really. Reeves isn't a hired gun director, but someone who very much likes to inject his own sensibilities into projects to make them as personal to him as possible, as witnessed by the shift in style and tone from Rise of the Planet of the Apes to Dawn and War (and Reeves' films just so happened to be much better too). It needs to be character driven, and that's harder to do when you're coming to a character with two movies worth of development - not to mention the fact this version of Batman has been doing it for 20-years or so - than it is when starting from scratch with your own complete idea.
Reeves didn't mention names, but he did speak of wanting to tell an emotional story with the character, along with wanting to get back to the detective roots, telling a point-of-view, noir-tinged storyline that would really emphasise the focus on the character's billing as World's Greatest Detective. That's a great idea, and the kind that really ought to have been done on the big screen by now, but it also jarred with the original vision for the DCEU's version of the character, especially with Affleck no longer directing or writing the movie either.
Reeves is still being vague on what kind of actor he wants, but the various reports have all stated he's looking for someone much younger than Affleck - around the 25-25 bracket has commonly been mooted - as he looks for not only a dramatic shift to what the DCEU has done. but a real fresh start too, as he's made reference to it not being in the same universe, which just further distanced things from Affleck.