Apparently the reason the second cube took so long is because it picked up signals from the Borg left behind in the movie First Contact (which was revealed in the Borg episode of ENT).Darth Tanner wrote: If the Borg were interested in how the Enterprise performed the super speed jump that Q gave them that interest should have died once they assimilated Picard. Could be a reason it took so long for a second cube to be dispatched, their interest waned once they realised the Federation tech was not as advanced as they had observed in their first encounter.
Distance, resources, depends on the objective, etc...Come to think of it, does anyone have an explanation why they would use multiple cubes against planetary powers in the Delta Quadrant, but single cubes against a major power in the Alpha Quadrant (twice)? Does not compute.
It's been speculated upon in other threads, including previous pages of this one.