It may be more than just the fact that she has died. Remember, when it was trying to get Angel to commit suicide, it never came to him as Buffy, even though that would have probably been the worst thing that it could have done to him, especially if he didn't know that it wasn't actually her. The second time that Buffy came back, though, if wasn't a case of simple resuscitation. Her body had decayed, dehydrated, and generally deteriorated for several months, requiring a very complex magical spell to bring her back. Plus, according to Tara's tests, Buffy actually was different on a molecular level than she had been before, which could be translated as saying that at least some tiny portion of what constitutes Buffy in the real world didn't actually come back, so it was magically replaced by something ALMOST identical. (Sort of like what happened in the "Jurassic Park" movies, and was explained in much greater detail in the novels.) That tiny difference might have been what allowed the First Evil to duplicate her in Season 7, but not in Season 3.