actually, they don't. dat's y all the remakes, reboots and re-imaginations.
any kind of adaptation from another medium is usually hot potato.
Different media, different writers. Never mind the blond hair, the trenchcoat, these are all but superficial (I do not even care about the lack of chain smoking). The writers for Hellblazer were what made the real difference. There are good batman comics (frank miller, grant morrison), and there are crap ones (most of the rest). I'm not gonna blindly read all of them just because it's batman.
One can only survive on comic book hype for so long before one gets called out for how shallow one's series be.
The series always seemed more like an adaptation of the movie Constantine rather than of the graphic novel Hellblazer (in fact, the movie was based on a Garth Ennis plotline and was arguably a more faithful adaptation of the novel).
Having said that, US dramas/sitcoms have a tendency to last way beyond their expiry date. Constantine will probably drag on into Season Two. Gotham will probably last at least three seasons, as will The Flash. Sad but true.