Dolittle: because the current interviewee chooses the next one from the list of most voted ones.
Someone suggested that current interviewees select the next one and I went along for a while. But then I started thinking that I could risk making interviews for the same group a friends for a very long time... and it's easier to collect questions for people that others are curious about than it is for people who could be known by only a few.
This way the community decides who they want to see interviewed and the current interviewee still has a word to say. And, if for nothing else, it helps to have a list of people to choose from in case the current interviewee can't make a choice for him/herself.

Sorry if this looks like it's being made up as I go long but I haven't been in the job for that long... and so I'm really making some things up as I go along