Considering that Picard was violating orders just by being there, how was he able to take command of the fleet so easily? His own crew thought the order keeping him away was bullshit; did the rest of the fleet think so too?
On a related note, why didn't Starfleet transfer Picard off the Enterpise and send it into battle under Riker's command? Riker is, after all, the only Starfleet officer in the Alpha Quadrant who has ever commanded a starship in a confrontation against the Borg and won. (Unless you count Archer, but my guess is that he's no longer active in Starfleet. Did anybody in the 24th century ever try omicron radiation?)
On a related note, why didn't Starfleet transfer Picard off the Enterpise and send it into battle under Riker's command? Riker is, after all, the only Starfleet officer in the Alpha Quadrant who has ever commanded a starship in a confrontation against the Borg and won. (Unless you count Archer, but my guess is that he's no longer active in Starfleet. Did anybody in the 24th century ever try omicron radiation?)

But, my personal interpretation, the Borg want technology as well as assimilating species. Earth in the 24th century is a greater prize than Earth in the 21st. So they send a cube to assimilate Earth, and only when that fails do they attempt time travel, a back up plan so they get something rather than nothing.
. Heck wouldnt it be better sent 2 or 3 cubes to assimilate Earth or just hold one cube at Federation border and if the Cubes fails their mission. Cube at the border would time traval back in time to prevent First Contact and sent message to Borg Collective.. Also Borg have Time Travel Technology to use whenever they want... They can try again and again until they suceed.