Ben Affleck, as recently as January 9th on Jimmy Kimmel Live, asserted in no uncertain terms that he was going to direct the solo Batman movie. That's after the dust-up over his comments to The Guardian about the script needing to be "right" for him to proceed.
Then Live by Night happened. The limited release in December didn't garner a lot of critical praise or audience buzz, but everyone was hopeful that things would turn around for the wide release. Nope. It's a bomb. Possibly a $75 million loss for Warner Bros. Whether the poor performance shook up some of the studio's confidence in Affleck to handle a blockbuster budget, or if Affleck himself took it to heart and decided to drop out, we may never know. I think it's safe to say that Live by Night did have an effect here. You don't lose $75 million for a studio and come out of it unscathed.
Now, Ben is out of directing Batman. He'll still star in it, and he's supposedly still working on the script with Geoff Johns, but Warner Bros. now has to get someone else to direct. Is it a sure bust? Not at all. We saw with Ant-Man that you can replace a director and move forward with a movie that makes a profit. What this does is further cement the idea that Warner's DCU movie development is kind of a cluster. Sometimes that adversity makes great art, and sometimes you get Suicide Squad or BvS.