From
the previous I have the thought that a lot of handwaving in fic is caused by a mismatch between how we read it and how we write it. Cos, see, we're all aware of the theory that one should always justify the emotions and set up the scenario, but once you've written your twentieth fic with the pairing, you really can't be bothered anymore.
This is especially true if you need to handwave something massive like half of your pairing being departed, dead, married to someone else, in prison, evil, disembodied, intangible, or a member of the Conservative party. The first few times you'll probably go to effort to justify your choices and how they manage to be present, but after that... well, you have already done the work, just not in that particular story. To go back momentarily to the talk of post-War Dromana's ever-increasing fanon content just for an example, it's kind of a choice between having fic where 90% of it handwaves the "How, exactly, is she not dead?" and having everyone writing the exact same story every time. And Dromana seems to be hitting the point where the first wave of writers give up going through the explanations each time. After a while it's just getting in the way.
Thus it be also with the Doctor/Jack and Doctor/Rose reunions, where it's either skip that bit or try to find the one remaining new take on it. Meanwhile the reader has not been following your fic career in sequence and will be reading a lot of what fandom as a whole is producing. After a while you just give up and assume they've already read that story, no?
Breaking up an OTP to get yours together? Effort. Lots of effort. So you skip on to "A was magically Not There, so B and C done sex." Especially if one is writing short one-shot fics, where your output can be fairly high and avoiding repetition quickly becomes a goal. You don't really have the space to go through the motions each time, but that can be annoying for the reader, so what's the best approach?