C.E. Evans wrote:
But it's not really inconsistent with Starfleet's mission at all. There really wasn't much information about the Borg for Starfleet to go on at the time, except second-hand accounts from the El-Aurians (who may not even have known where the Borg originated from or where they vanished to over seventy years earlier).
Exactly. And Starfleet's resources weren't infinite. Like any other organization, they'd have to prioritize certain missions and projects to which their resources would be devoted. It's simply not good decision-making to utilize significant time and labor to chase down a centuries-old rumor without any current evidence that might give the rumor credibility. Now, once Picard and the Enterprise
saw the Borg for themselves, it's possible someone at Starfleet made a connection and began devising solutions to the Borg problem. For all we know, that person may have been none other than Elizabeth Shelby, who was so important in helping thwart the attack on the Federation the following year.