why doesn't this automatically trigger a by-eleection?

I'm mainly talking about this http://news.bbc.co.uk/1/hi/wales/wales_politics/8401427.stm though I appreciate that not many people here are bothered by Welsh politics.

I also find it a bit bonkers that someone will one day wake up and decide to leave a left wing party for a right wing one, how can you have any credibility with your constituents?