My understanding was always that he found the shooting schedule of a TV show far more taxing than the film work he usually does. I don't know how valid that reason is, he had done telly work before and much as I liked him as the Doctor, it's hard to see how far the survivor guilt Doctor could have been taken.
My gut feeling, which I have no real evidence to back up, is that RTD got a big name in to help relaunch the show, but the intent was always to regenerate him after a year with someone who was more classically Doctor like. Like I say, I have no evidence for this and I'm sure if Eccleston had found he loved the role and wanted series 2 RTD would have jiggled his plans around.
My gut feeling, which I have no real evidence to back up, is that RTD got a big name in to help relaunch the show, but the intent was always to regenerate him after a year with someone who was more classically Doctor like. Like I say, I have no evidence for this and I'm sure if Eccleston had found he loved the role and wanted series 2 RTD would have jiggled his plans around.