Well, first of all, by the time we get to FC Starfleet has had several years to develop new weapons and tactics for use against the Borg. They've known since The Best Of Both Worlds that it was just a matter of time before the Borg attempted another invasion.
Even so, however, the Starfleet weapons don't seem to be making much of a dent in the cube until the Enterprise-E shows up. It's not until Picard takes over the fleet and orders everyone to concentrate fire on what he knows to be a weak point that the cube starts taking heavy damage.
Precisely. Picard's prior experience with the Borg made destroying the cube much easier than it would have been otherwise, ironic considering that Starfleet didn't want him to be part of the fleet for that exact reason.