star trek - Why did Admiral Janeway choose to travel back in time to 2378?

In the series finale, Admiral Janeway travels back to 2378 to take Voyager back to Earth. Out-of-universe, of course, this point in time was chosen to fit in with the continuity of the rest of the series. In-universe, however, it seems that Janeway could have chosen any point in time, since she had the ability to travel to and from the Delta Quadrant at will. Is there a reason given, in-universe, for why she chose that particular incident to travel back to?

Answer

Janeway appears to have chosen 2378 for a couple of very specific reasons;

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  That they'd been traveling long enough that Janeway had begun to take a morally ambiguous stance toward Starfleet regulations, especially regarding the Temporal Prime Directive. She's unwilling to hear much about what happens in the future (at least at first) but with just a little convincing she's willing to fit the Voyager out with advanced technology and take on the Borg to get her crew home.

  
  



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  That this date represented a turning point of regret for Janeway. After making it back to the Alpha Quadrant she probably had a good long think about her actions, chief of which was her decision to reverse away from the nebula, having later (presumably a decade later) found out that she'd been mere minutes away from a portal straight back to Earth, one which would have bypassed the Fen Domar and the Borg that they would have otherwise encountered on their way back... if only they'd had some way to defend themselves against The Borg...

  
  

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Note also that in another time-travel episode she was unwilling to sacrifice the Voyager's impact on the Delta Quadrant. Again, it may well be that from this point onwards that their involvement was pretty peripheral, just flying and getting shot at.

TELEK: I can assure you, Captain, that I would not do anything that might contaminate the future and perhaps harm the Romulan Empire, but, in twenty years I could alert Starfleet not to launch the mission which sent you here.

CHAKOTAY: I'm afraid that's not possible either. We've already had a huge impact on this quadrant. People and events here would be drastically affected.

_{^{VOY: Eye of the Needle}}

Also, if she went back further (and, for example warned Starfleet not to launch the Voyager in the first place), she might endanger Seven-of-Nine's rescue from the Borg and negate the existence of Naomi Wildman, two people that she pretty obviously views as surrogate daughters.