Yep I saw that the other day and those spoilers would go along way as to explaining why. He didn't want to come back for Arkham City but did so because of the fans. I think Kevin Conroy said that the fans were a big reason for him coming back to do the game as well. I will miss Mark's Joker as his was my favourite version of the character, Heath's version is my second favourite.
Hamill announced this back in 2010, not long after Arkham City
was announced. Don't forget that Hamill is 60 years old, and by this point the Joker voice is absolutely killing
his throat. Saying that he did Arkham City
because of the fans is false, however -- Hamill had originally planned for Arkham Asylum
to be his final go as The Joker, but after reading Paul Dini's script for Arkham City
, he felt that he had to return for one last go.
As for Conroy, that dude will continue voicing Batman as long as WB wants him and his voice allows. Just a few weeks ago, he was announced to be playing Batman in Justice League: Doom
Also, "Mark?" "Heath?" Really? Are you on a first-name basis with these people?