Actually Robert Kirkman said that actress who played Sophia didn't want to die and was very upset. And BTW do you think any kid who want to be an actor/actress wanted to move on from such popular show? I don't believe in this.
Okay, let's put it this way then...
When they change the way the characters is put are going basing, when they change the way the story plot is going by seasons play. And the performers of the show/series, don't like their characters to be change at all or the storyline plot by seasons also at all. They've the right walk off the set and never come back. It has nothing to do with high population of the program at all. It's more on them becoming more popular into their acting career. You don't really become popular staying on one high population tv program. You need grian more work for becoming popular than the one you've appear on previous. And The Walking Dead isn't no Comedy Sitcom or Family Drama. It's more serious on many levels being a drama, action, horror. You say the actress plays Sophina was upset? Well, I don't blame her at all... I would be upset to, they went ahead change my character & story on me.