I always thought this was a response to how successful Joker was. Yes, it was planned to have a Batman solo movie before Joker, but at the time, Affleck had already been attached and there was no need for a "reset". With him leaving the character, you have to wonder if they had a conversation about continuing the storylines as in Justice League or go in a different direction.
They are obviously going in a different direction since Batman is young again and not an old man like in Justice League. And my guess is they directed Matthew Vaughn to do something similar to what was done with Joker as soon as that movie became a surprise success.