I've always thought that the 'message' sent by the ENT Borg was what would eventually bring the Borg in "Neutral Zone", although it is kinda strange that they didn't give the Collective all the data they had about their encounters with Earth and their outcomes. Or maybe they did, and this is all part of some scheme by the Collective! (and why did the drones go running for home? a handful took over several humanoids and their ships; i would have stayed on Earth and go "Romero Zombie Movie" on them!)
Just a couple of ideas to toss into the ring:
1) 7 of 9's parents were chaseing the Borg, going all 'Jane Goodall' on them. I'm not exactly sure how long ago, since I'm not quite sure how old they said 7 was, but maybe they were one of many 'Borg hunts' sponsered by Phalanx (fun fact that you might know; the Phalanx was a Techno-Organic race that was defeated EXACTLY like how the Borg in First Contact were!)
2)I know it doesn't fit with the "Utopian Ideal" of the 24th Century, but maybe the Borg info was hidden much like the alien ship data in ID4; "complete deniability". Or maybe the budget/resources was cut off just prior to "Q Who", since there had been no signs of 'Archer's Cyborg Boogymen' for almost 200 years!
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"Yes, it's the Apocalypse alright. I always thought I'd have a hand in it"
Professor Farnsworth