Really only know him from Cracker, where he was part of an ensemble, not the main man. Wasn't it the same in his other roles? It's not so much the effort, but DW is so heavily built on the two principals, whereas in Cracker that was only Coltrane, CE probably was only in about 25% of the time (except when he died). I suspect the combo of being in over 80% of the show, plus feeling responsible for all of it (would anyone have said it was Piper's fault if it had failed? She was teenie popstar with no credentials), and the family issues all counted.
It's a shame they didn't convince him to at least start s2 though.
As far as Tennant, unless his career took off hugely or Eccleston stayed for 4 or 5 years he'd have always been the Dr. After all, he'd worked with RTD before, he is a fan and probably works better tbh, he's more child-friendly not sure how, he just is.