Other Than His Massive Fan Base, He Has Nothing for him in the Realm of Comics... I Mean, he kills people at random and Should have Been killed Himself ages ago. 
Think about it.  

Captain Boomerang Orphans Tim Drake - He Dies. 
The Green Goblin Murders Gwen Stacy - Dies. 
Maxwell Lord Shoots Ted Kord - Die... etc, etc...  

Why Does Batman Fight For this Freak's Life? Jason Todd deserves the Honor of Taking him out, But For Some Reason above all of Jason's Other Victims, Batman Protects the Joker from Death. Why?

Because as Batman Beyond showed, other nastier freaks just come along.

  Because Batman is a vigilantee and he doesn't believe in taking the law completely into his own hands. 
 
  Also, apparantly, the Gotham City justice department doesn't support the death penalty and, also, is completely incompetent. 
 
  By the way, this topic has been done so many times, I can't even count the number on my fingers.

As MrDelicious says; money. 
 
Do keep in mind that we are talking about comics, and not real live (I know, sucks right?). The Joker isn't killed because Batman doesn't want to kill, or because he is insane and thus not chargeable for death penalty. 
It's because he brings too much money in the pockets of DC and he has a to big of a fan-base, as you yourself already concluded. 

Joker has been killed before.
 
Read Dreamwar. It's non cannon but Apollo cut him in half with a laser beam.

" His Massive Fan Base "

He's got years of character development , a massive fanbase and he's universally seen as one of the premier Batman villains, if not one of the greatest villains in fiction. That aside if he died the fan backlash might destroy Eastern Europe. 

Money, money, money, money..... money!

Absolutely because of money & his fan base, although originally (during the Golden Age of comics) he was supposed to die in his 2nd appearance but the editor at the time spared him. I'm sure the editor's reason wasn't money (& Joker had no fan base). Someone saw something in the character, and after enough appearances other people saw something in the character too. I do think after what Joker did to Jason Todd (A Death in the Family) & Barbara Gordon (The Killing Joke) he really deserves to die, but because of Joker's exposure in movies (first in '89 and now recently in '08) the character is just much too popular amongst mainstream comic book/media fans to be killed. I also forgot to mention how influential a role Mark Hamil played in making the character likable to children, especially when paired with Harley Quinn who is very popular amongst anyone who grew up watching BTAS. Despite how evil Joker may actually be written to be he is still likable enough for general audiences unlike characters like Zsasz & Black Mask for instance who are purely evil & aren't actually likable/charming/funny in any way. If you look at other popular villains throughout bat history who will also not get killed such as Catwoman, Penguin, Riddler, Mr. Freeze, Poison Ivy, & Scarecrow you'll see they also share what the Joker has; personality & character to go with their madness. But I do agree at a realistic literary level it is very hard to suspend your disbelief on how no one has killed a mass-murderer like Joker. That's why some people may hate the character, they just find it more interesting to invest time in a Batman story that isn't one-dimensional.

"Money my good man, money and marketability. "

Because killing people is bad?

from a comic perspective, if batman lets someone kill joker then joker wins because he still broke batmans morals. but if joker goes his whole life trying to tear batman down and batman never lets him, then the joker loses. now if joker died without batmans knowledge then it would be a different story, but when batman doesnt know were or what joker is doing, then no one does.

" @MrDelicious said:

"Money my good man, money and marketability. "

QFT "

"@Tudoh: Batman does the same thing, and he died recently. We can have Joker die for a little bit "

I think a better question would be:
 
Will anyone ever supplant The Joker as Batman's greatest villain?

" I think a better question would be:  Will anyone ever supplant The Joker as Batman's greatest villain? "

Batman comics just wouldn't be the same without him.

1) What if Gotham was all just a never ending dream in the head of a comatose Bruce Wayne & the Joker was his evil inner child?
 
2) Massive fanbase. Women prefur Joker over Batman. Women read comic books.
 
3) He can't be hypnotized, he's immune to fear gas, his blood is poisonous, he's immune to toxins & poisons, he's smarter than the Riddler, he's an actor & an evil detective who fakes insanity & is too dangerous to be kept in a normal prison, if he gets injured, he can still move acrobatically.
 
4)He panders to the fans who dig gay evil sadomasochists; a common enemy type in anime.

All three examples OP gave are back alive :P

No one has killed the Joker because when you kill the Joker, Kingdom come happens.  That didn't work out all too well, is that really what you want? :P jk

But its not been mentioned so I figured I will do it, he has been killed before, by Nightwing.

"@batman_is_god: I Like That Idea.  @entropy_aegis: Sinestro's died before... And Red Skull's died about five times at least. "

"But its not been mentioned so I figured I will do it, he has been killed before, by Nightwing. "

Well I've looked at some of the other posts by people and they aren't wrong, but I think you're looking for an in story reason. At the end of the day, if you kill him he wins. I mean that sounds weird but the guy is sucidial, but theatrical, and extremely philosophical. He is a nihilist and more or less wants to prove that life has absolutely no meaning. Batman is on the exact opposite of that view point. Batman's biggest claim that he is right is that he doesn't need to kill to keep on going. By killing Joker Batman loses all ground and validity to his claim. Now as far as anyone else killing him, Jason doesn't want to do it himself, he wants Batman to do it (Red Hood Lost Days number six), and a lot of other people have tried, but Batman's always been there to stop them, and not stopping a murder that you can is more or less just as good as murder. This position is not to be confused with letting someone die, like Ra's on the train in Batman Begins (allegedly). As far as some random person, go to Google and look up "Batman City Of Scars" otherwise, I'd say the bastard is really intimidating and the idea of someone just killing him may sound good in theory but shooting someone isn't anywhere near as easy as it sounds...

Because Batman won't let anyone kill him, I like how the depicted Batman's view on killing the Joker in the Under the Red Hood movie. But in truth if anyone killed him, it would be Batman, it has to be him or it just wouldn't be right, y'know?

Because he'll come back.

You know, because comic books.

Its a shitty excuse but they cant kill him off because fans adore him, that is the sole reason no matter how you justify it.

Because if he dies nobody will be able to make this exact same thread anymore.

The idea that Batman won't let someone kill a man who tried to kill his family is just...ugh. Btw, I think they should have toned Joker a bit from mass murder to sadistic prankster who mostly crippled and scarred people, occasionally killing.

@joygirl: HAHAHAHAHa he he he heh heh heh heh ho ho...ho....