In reality, it's because TNG came out well before Voyager was even thought of. But an explination from a in universe perspective: The Raven was swept away to the Delta Quadrant, any research they had was lost with them. At the time they began their research the Borg were little more than rumors passed along. A better question is why they didn't know about it when Captain Archer encountered them on his ship.It all has to do with the affects of using FTL and the first Cylon invasion.
Cylon? Isn't that Galactica 1980?
It's safe to assume Starfleet locked away all the notes somewhere in a vault until they got the Enterprise-D report then dug them up, decided they were useless, especially the 200 year old reports by Archer, and threw them away. ----------