When the El Aurians were rescued they may very well have told Starfleet everything they knew about the Borg, but when Starfleet reviewed the facts they likely felt that the Borg are on the opposite end of the galaxy and won't pose a threat any earlier than several decades later, it's not an immediate problem. We got other things to worry about, like maintaining this fragile new peace with the Klingons. Not to mention the Romulans seem up to something.
Or, alternatively, maybe reports of the Borg did stir something among the Starfleet brass who put forward several initiatives to counter them as soon as word first came from the El Aurians in the 2290s. But then years go by and then decades with no contact from the Borg and no evidence that they are sizing up the Federation or even that they exist. Eventually someone's going to realize that there's better things to be devoting resources to. Like cleaning up after the Tomed Incident, dealing with Tholians attacking starbases, war with the Cardassians, peace with the Klingons deteriorating, some mysterious race called the Ferengi who are rumoured to eat people and have destroyed a Federation starship commanded by one of Starfleet's top captains. In light of all this, one can understand why Starfleet and the Federation would downgrade the priority assigned to what is starting to look like a space myth.