Re: Why didn't the Borg...
In "Dark Frontier"'s case, it could merely be due to the planet being in the Delta Quadrant relatively close to the Hub, where there would be multiple Cubes available.
The transwarp conduits appear to be limited to single ships at a time, unless there's a scene with a Borg fleet going through I'm not remembering.
Dr. Howard, Dr. Fine, Dr. Howard: For duty and humanity! --Men in Black (1934)