I have also seen those who have refused to move to campaigns with better rates, they will be picked if they applied because of their good posts. They choose to remain. Is it about loyalty? Is it that they are scared of switching? Is it about fulfilment with where they are?
Each member will have their reasons to change or not change from one campaign to another. Personally, I like the idea of being loyal to a campaign, since they have been being loyal to me as an employer, week after week, for years already, without failing in anyways. I could try swaping to another campaign for superior financial purposes as soon as they open a spot, like many members have already done, but why to seek for a change if the currently situation I find myself in is fine? I think that would be a greedy move from my side, which could end very badly after all, since I would be exchanging a long lasting, solid and loyal campaign for a brand new one which can last for few weeks or one month...