Why did Christopher Eccleston only agree to do one series of Doctor Who?

Answer 1

There are mixed reasons as to why he left. According to his Wikipedia article, in 2010, Eccleston said himself that he "didn't enjoy the environment and the culture that [they], the cast and crew, had to work in." Russell T. Davies, executive writer for most of the series, stated in 2008 that Eccleston was only signed on for only one year because it was uncertain whether the revival of DW would have been successful.

There's also rumors that Eccleston didn't want to be solely associated with DW (e.g. we associate David Tennant with the Tenth Doctor, Matt Smith with the Eleventh Doctor, and so on), hence his leaving from the show.

There are other rumors floating around out there, but only the facts in the first paragraph are true.