First time: Because I had found a career and I didn't feel it would improve my options. Second time: By that point I'd decided that I didn't want a career in research and instead could make a more enjoyable career in support. Third time: I already had a good job in support and didn't want to continue in research. Fourth time: I believe I've provided a satisfactory answer and I'm not sure I understand what I would have to add by answering the same question again.