Why did Christopher Eccleston leave Doctor Who?
No one knows for certain. There are a lot of different rumors going about. One was that he was exhausted from overwork on the show, which is very demanding. Another is that he had only been given a one-year contract because the powers that be at the BBC didn’t know if the revival of Doctor Who would be a success or not. Another is that he did not really enjoy working on the show (that might be supported at least in part by the fact that he generally does not like to talk about his role as the Doctor with the media). Frankly, we probably will never know the true reason as everyone affiliated with the show seems to want to put their own spin on it.