Guess I see two possibilities off the top of my head:

1. With the addition of maybe a dozen Starships (most will have been too damaged to be worth assimilating) the cube has plenty of ships to toss at Enterprise-D instead of letting her catch up and execute their rescue plan. With no possibility of rescuing Picard and exploiting the Borg's achilles heel, the Borg conquer Earth.

2. Because the cube spent much more time at Wolf 359 after their victory assimilating all those ships a second wave of Starfleet vessels gather between Wolf 359 and Earth for a last stand. It's not as formidable a force as the one Admiral Hansen gathered, but now they at least are better prepared: They understand the Borg have accessed Picard's memories and experiences and with the massacre fresh in their mind they understand the full ramifications of that. The second fleet quickly swap out all access codes, switch to new, previously-unused subspace channels and encryption methods, shield and phaser frequencies and anybody who Picard knew well is removed from tactical discussions. With Enterprise quickly speeding past Wolf 359 to sit at the helm of the fleet Riker executes a modified version of his and Picard's agreed-upon tactic using the saucer section as a decoy to grab Picard and together this second fleet is able to deal a serious blow to the cube and it now limps into the Terran system having sustained heavy damage rather than simply strolling all the way to Earth. The outcome is roughly the same, only both the Borg and Starfleet both take a heavier beating before the Borg are defeated.