Earth will fall because, despite there being a space battle only 200,000 Km away, Starfleet security took no action to intervene. Earth will fall because, despite having a defence grid (mentioned in ST09 but also seen in TMP) nobody bothers to activate it to stop a ship with only one life sign on board from hitting a city and killing thousands of people. Earth will fall because people that stupid deserve everything they get.
Since you mentioned TMP; were was the fleet in that one?
If you go by what's said in the movie, they are far enough out that nobody can get recalled fast enough to intercept (this is in part because vger is coming at a sustained speed - 7.6? -probably higher than that of starships, which had a top cruising speed of warp 6 and were supposed to start shaking apart if you ran them too long at warp 7 or 8.)
If you figure there are still only a dozen or so starship-class available (which is debatable), there's a lot of space to be covered, so it is at least somewhat plausible that this is the only starship
in interception range.
An earlier draft had vger intercepted by a light cruiser that managed to get in its way, and it basically was replaced by that gorgeous Epsilon 9 station in the movie.
Yeah - there are ships and there are starships. It is a bit silly that there were no ships at all between Klingon space and Earth but I have to respect TMP for treating space as the truly vast and terrible beast it really is.
I can imagine a starship crew out on the rim, exploring all alone, with no help - like in Alien (I even did an Alien/TMP crossover - lol). I can't imagine that in NuTrek.