1. Because Frank Miller Said So
In 1986 DC released a four part mini series by Frank Miller called The Dark Knight Returns. At the time the character of Batman had become somewhat stale with very little of DC's output for their top cash cow challenging the readers who were turning to Marvel in their droves. Miller's work was like nothing that had come before it and completely changed the character forever. In the story Batman is long retired, out of shape and out of sorts with the people of Gotham City. His big return to the world of crime fighting comes at a time when violence has besieged the world and it will take a very different kind of Batman - a God damn Batman as it were - to restore order in this very different world. Batman's re emergence also brings back his arch enemy The Joker who has been in a catatonic state in Arkham ever since Bruce Wayne called it a day as a 'hero' further hammering home the idea that without a Batman there would be no criminals. Unlike previous incarnations of the masked marauder of Gotham this was a Batman with an edge unsurprising with the creator of Sin City at the helm and this edge more than anything else shaped the character into the ultimate anti hero that even Superman wants to put an end to. Of course you may argue that the work of Frank Miller is the exception and not the rule and instead the traditional Batman stories are canon and thus the only ones that should be considered when making this argument but tell me what writer since 1986 has legitimately not been inspired by Miller's characterisation. I think you will be very hard pressed to find one. SO now all that is left is for you all to debate amongst yourselves. Have I done enough to convince you or are you even less convinced? The beauty of our combined love of comics, Batman and the Geeky little asides like this is it brings out the true passion in us all and I look forward to reading all of your comments. The floodgates are open Bat fans. Let the debate begin.