Every batman fan would say "cuz he is the goddamn batman".
This post does not compare the powers or greatness of the superheros with batman's, but about the inception of the comic series and the way the series is headed.
1. Any other superhero has one or more special powers, and those special powers are within the boundaries of average human brains thinking.
In other words these special powers are fantasies of any average individual, nothing new, nothing special, making the superhero himself inhuman and over powered.
And to create a story, they create some super villain who also has super powers. What's common to all these series is that they concentrate more on the materialistic part of being a superhero, powers, bam bam, blasts, common people dying etc., than the nature and thinking of the superhero or the villain.
On the contrary, Batman has no special powers. Whatever he has to do, whatever he has to accomplish, he has to do with the limitations a human being has, which makes us wonder how he is gonna do it. That also makes us love him the way he finishes the work(defeating a villain).
Batman knows his limitations and work according to them, taking the help of cops and citizens of his city.
2. Unlike any other superhero who fights super villains, batman is against the evil nature of humanity itself. He considers the nature of the villain into account and tackles him a way no superhero does. He even chases villains and mobs while being chased by the cops of his own city.
3. The other difference is, in any other superhero film, the introduction of villain is done by the death of an average human being and more casualties occur in the later stages of the film. It's like the writers doesn't give a f*** about the lives of normal humans. Human beings are shown as useless in most of the superhero series.
But in batman, he values life of every individual. Remember Batman do not kill. He helps them and takes all the help he can get to subdue evil.
These all and with many more reasons make the people love dark knight the hero of the comic.