You'd think that availability wouldn't be an issue for animation, true, but sometimes it can be. The reason Justice League
had George Newbern replacing Tim Daly as the voice of Superman was because Daly's starring role on the TV reboot of The Fugitive
(which entailed a lot of travel) made him unavailable for voice work. And I've read about other voice roles in other shows being recast due to the unavailability of the original actors. Although sometimes it's just a matter of becoming too big a star for the show to afford. On the '90s Spider-Man
cartoon, the original voice of the Scorpion was Martin Landau. Then he won an Oscar for Ed Wood
, and suddenly he was out of their reach. Richard Moll took over the role for the rest of the series.