A good point. I always wondered why TNG only ever used one Borg cube in their stories, even in the First Contact feature. At least Voyager tried something different with multiple cubes and so on making an appearance at various points.
in first contact I think they were testing the federation. It took all the might of the fleet to destroy the one cube, and only because Picard knew a tactical place to aim. Now the Borg know that one ship is a threat, and five would be completely unstopable.
and in all fairness, why would they borg want to assimilate the federatin? they already know everything about them having assimilated Picard and various other officers, the federation's technology is not more advanced, so the borg have nothing to gain by going after the federation