The story as I recall it at the time was that Sarah was supposed to reprise her role in "The Girl In Question," but was still tied up in Australia filming "Scooby Do" when the time came. Rather than drop the episode, Joss made the decision to use a stand in for Sarah and go ahead with it that way.
My guess is that by the time that Joss began writing Season 8, there had been so much uproar by fans about the fake Buffy that he just decided to go with it in exactly that way, which is how the "Buffy" in Rome actually became a fake. Also, and this is just my personal theory on the matter, I think that, while the character of The Immortal was so interesting as a foil to Angel and Spike, all charming, wealthy, powerful, a legendary lover, and so on, that, by the time that Season 8 actually rolled around, Joss found that he had created a mystery character of such legendary, mythic proportions, that he simply wasn't capable of portraying that being in the flesh, so to speak. So, rather than even try, he mentions Morty once in passing, in Issue 1, then forgets all about him. Either that, or the character who he imagined turned out to be so close to one of the characters from Greg Widen's "Highlander" universe (probably Methos) that he decided not to use him for fear of copyright infringement.