I hope that by the time this series is over, there will be an explanation for why the Borg decided that the Federation needed to be exterminated, rather than assimilated like in the past. It would have made more sense had the Einstein reached Borg space, but we know that didn't happen. Picard himself in 'Gods of Night' wonders this same thing, but he does not reach any conclusions as of yet.
Actually this was foreshadowed/addressed in Greater Than the Sum
. Admiral Nechayev said that after Voyager
destroyed the Borg's transwarp hub, that elevated the Federation from a minor annoyance to a major threat in the Borg's eyes, and the loss of their transwarp access to the Alpha Quadrant was the only reason they hadn't already
destroyed the UFP in retaliation. So their desire to wipe us off the map has existed for about three years at this point -- it just wasn't until now that they found a way to reach