As for why others aren't brought back, maybe some of the ones they'd like to ask back are no longer interested, because they're at different places in their lives that make it less possible (or even less attractive) to them. I'm always amazed that people can manage to get 39 days off of work once to go on Survivor. Getting it off multiple times might be too difficult. Or they might be running their own businesses and be reluctant to be out of the loop for 39 days. Or they have family situations that make leaving for that length of time untenable or they've developed health issues that make playing Survivor more of an issue for them than it would have been earlier. A million dollars is a lot of money, but if other things are at stake, some people may be willing to forgo it. Or heck, maybe some of them feel like they've been there, done that and have moved on.