Zim shouldn't have gone off the air just because Whats-His-Name did another cartoon voice also.
ZIM was taken off because Nick didn't like the show. Though it did bring in ratings, they were not from the target audience.
Also add in the fact that Jhonen Vasquez (the creator) was getting annoyed/frustrated at the limitations and restrictions Nick put on the show.
He said, he MIGHT like to work on it again, provided he was given more freedom with it. (I'm sure you're aware his comics are quite gruesome and filled with dirty language.) Though, another key staff member, Steve Ressel (the director), said he does not want anything to do with that show, so the show returning is fairly unlikely.
What you said about someone doing another voice had nothing to do with the show's ending. I'm assuming you're talking about Richard Horvitz, who was pre-occupied in doing the voice of Daggett in Angry Beavers when ZIM was still in the developing stage. Nick said they didn't want someone to do the voices of 2 characters in different shows. That's why, in the unaired pilot, Billy West voiced ZIM. The Angry Beavers had ended, and they called him back in to voice ZIM (he had tried out for it earlier, obviously).
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