If we're not going by what T'Girl said (Which makes a whole lot of sense, thinking about it) then the next plausible explanation is:
As they said in 'Regeneration,' Cochrane told people of the Borg, but no one took him seriously, and brushed it off to being a drunken ramble. Then, when the El-Aurians are found in the Nexus ribbon by the Enterprise-B, they are apparently fleeing from the Borg (Why Guinan is on board those ships when they were apparently fleeing the Borg is beyond me, considering she was on Earth in 1893.) Any number of reasons could be thought of on why Starfleet didn't exactly take notice of the fact that they'd probably tell them that the Borg destroyed their homeworld. Maybe some admirals thought that it wasn't important enough, and it quietly fell away.
But then the Hansens are introduced, which sort of complicates matters. If some (quite possibly insane) scientist could dig up facts about the Borg, why couldn't the Enterprise-D do the same when they encountered them in the events of Q-Who?
As I said in the beginning of this post, I'm inclined to believe that T'Girl's explanation is the most plausible.