Because it's probably the most fun, blatant piss-take on Batman and Miller's own success in years? If you can't laugh at yourself
The comic is a commentary on how people are roped into buying anything with a big name attached to it, then when they look at the dazzling artwork and generic one-liners direct from Sin City, the target audience All-Star is poking fun of will keep swallowing it just to bitch and complain rather than enjoy it for what it is.
And Superman was always a worthless tool outside of the movies unless someone good wrote him, heck "Tower of Babel" saw the entire JL fall thanks to the use of traps Batman created to kill them if they went renegade. Whilst it was'nt Batman's doing, the traps proved the DC's most powerful are useless against a common individual, and that's why people love the character and Miller's version. Who the **** wants these guys running around when there ticking time bombs and in Supe's case, wll kiss anyone's ass if there in a posistion of preserving national security
Batman is an ass-kicking, bitter, cynical motherfucker, a "real" character as opposed to the ham and eggs of the JL, look at DC's long line of current storylines, "Identity Crisis", "Infinite Crisis" and "52" have since helped to reinvigorate DC, by making the heroes and the universe darker, harder-edged, more violent, and more flawed. Something that's "real"