He's last generation, John and Bill are gone and he was on the slide. And let's not forget his "fiscal hole" blunder.. what a balls up. His political stock was toast, no point in sticking around (hint hint Gerry Brownlee, Nick Smith etc).
Actually experience and guidance of the (slightly) older generation is good, for both parties. Someone posted a clip on this thread the other day of Puala Bennett in question time. Incidentally it was back when Lockwood Smith was speaker. I was watching a debate the other day and Brownlee had to pull up a junior labour speaker three times for wandering off topic. They mightn't be as 'progressive' thinking as a younger MP, but they actually bring a bit of dignity and values into the House...sometimes.