the reason is the amount of viewers it gets. I think the Joker role will forever be linked to Heath Ledger.

You can say that a 10 more times. 1980s Keaton was the best. Johnny Depp has even made a career out of playing 1980s Keaton.

Another piece of the equation are characters that were originated by an actor. Characters that did not exist previously in any form and the public had no previous conceptions. Hence, the character's likeness is forever associated with that actor.
Harrison Ford-Indiana Jones
William Shatner-Capt. Kirk
If comics of these two characters were to appear 100 years from now, Ford's and Shatner's likeness would be used.

Couple of classic ones
Karloff-Frankenstein
Lugosi-Dracula

The Christopher Reeve association is easy to explain. He's Superman. For almost anyone of a certain age, there will never be another actor that compares as the last son of Krypton. He was born to play that role, it can't be done any better. He was a good actor that did a variety of genres, but nothing that resonated like his take on Superman and Clark Kent. It's that magical alchemy of actor and role that is difficult to predict in advance.

to make it harder he lived up to the title after his time as the character.