8 Reasons Why Christian Bale's Batman Is Better Than Ben Affleck's
4. He Was The Focus Of The Story

What really hurt Affleck's Batman was how poorly directed he was - the performance he gives, especially in the cape and cowl, is pretty solid, but because the two-and-a-half hour movie is so messily focused he never gets the chance to be the Batman (and as Arkham Knight said, that's what's really important). He may get the most screentime, but all of his scenes are tightly focused on his campaign against Superman, later revealed to be manipulation by Lex Luthor, and in the final fight he is the most dispensable player by far.
Batman is, quite simply, not the focus of the story. This is always to be expected in an ensemble piece - for all his screen time and top billing, he was always sharing the movie with others. Which is fine and could have worked in the movie, but it doesn't allow Affleck the chance to really stretch his wings and show he's a Bat to be reckoned with.
Obviously Bale was the undisputed star of all three movies, so has a distinct advantage here, but The Dark Knight Trilogy was in contast all about Bruce Wayne and his crime-fighting alter ego. And not just from a narrative perspective - all the themes were firmly rooted in the character too.