From his own mouth, apparently...
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At an acting master class, at the Theatre Royal Haymarket. Christopher Eccleston was asked why he left, such a high paid job like Doctor Who. He responded:
“I left Doctor Who, because I could not get along with the senior people. I left, because of politics. I did not see eye-to-eye, with them. I didn’t agree with the way things were being run. I didn’t like the culture that had grown up, around the series. So, I left. I felt, over a principle. I thought to remain, which would have made me a lot of money and given me huge visibility, the price I would have had to pay, was to eat a lot of shit. I’m not being funny about that.