An interview with CVG suggests that the move was intentional. Hamill described the attitudes he faced when he started working in games. "I'm sure there's a range of opinions, from 'You're slumming it', or 'Can't you get more legitimate work?'," he said. "But that snobbishness comes with the business." He relished the opportunity to merge his animation and game careers with a return to Joker's smarmy mania. "I thought this will be one last chance to play Joker. And it was so much fun to play a character who is clearly insane."
Here's a surprise: the reason Hamill hasn't performed Luke Skywalker's voice in one of about a million Star Wars games isn't because he's not interested in reprising the role (though he isn't): nobody ever asked him to. How has that never happened? So many games have been made based on the three movies that an established video game voice actor starred in, and it just never came up?