It might be related to financial aspects. If the sales of the graphic novel have been consistently low, the publishers may decide to end it. Another possibility is that the creative team has had internal disagreements and they can't continue working together on the project. This could lead to the decision to end the Batman graphic novel. Also, sometimes a series is meant to be a limited run from the start, and 'to end on' could just be the natural conclusion of that planned run.
One possible reason could be that the Joker has a plan that will cause mass destruction in Gotham and Batman has no other way to stop it. For example, if the Joker has rigged the whole city with bombs and the only way to stop the detonation is by killing him. Another reason might be that the Joker has kidnapped someone very important to Batman and is about to kill them, and Batman has no non - lethal means to rescue them in time.
Well, one reason could be that MHA is interested in different superhero universes and wants to explore how Batman's world is depicted in fan - made stories. Another reason might be for inspiration in terms of storytelling or character development, as Batman has a rich and complex character history.
Perhaps the story could revolve around Batman's legacy. He might be training a new generation of heroes or trying to ensure that Gotham remains safe after he's gone. He could be dealing with his own physical limitations as an old man while still having the will to protect the city.
One reason could be to explore a darker side of their relationship. Maybe the writer wants to show that even heroes can have moments of extreme frustration or anger towards their sidekicks. It could also be a way to shock the readers and make them think about the characters in a new and unexpected way.
One possible feature could be its unique art style. Graphic novels often have distinct visual aesthetics. Maybe it has a darker, more gritty look to match Batman's noir - like world.
Maybe to save the Justice League from a powerful enemy. He might sacrifice himself if it means the others can escape or defeat the foe.
One reason could be the inaccurate characterizations. Fanfiction writers might not fully understand the nuances of Nightwing or Batman's personalities, and Nightwing hates seeing a wrong version of them. For instance, if Batman is made too soft or too harsh in a way that doesn't match the established canon.
One reason could be that Batman is written as being too self - absorbed in the fanfic. He might be so focused on his own mission that he doesn't want the distraction or responsibility of taking in Robin.
One reason could be that Bruce Wayne never had the traumatic event that led him to become Batman in the original story. Maybe his parents' death was prevented somehow, so he never felt the need for vengeance and justice in that extreme way.
One possible reason could be that the story was not engaging enough. Maybe the plot was too predictable or the characters were not well - developed.